The data is taken from a database view created in the file addons/point_of_sale/report/pos_order_report.py. Also the xml view for the model is given in the file pos_order_report_view.xml.

The model report.pos.order gets its value from the database view defined in the init function of the model. It is shown below:

class pos_order_report(osv.osv):
_name = "report.pos.order"
_description = "Point of Sale Orders Statistics"
_auto = False
_columns = {
    'date': fields.date('Date Order', readonly=True),
    'year': fields.char('Year', size=4, readonly=True),
    'month':fields.selection([('01','January'), ('02','February'), ('03','March'), ('04','April'),
        ('05','May'), ('06','June'), ('07','July'), ('08','August'), ('09','September'),
        ('10','October'), ('11','November'), ('12','December')], 'Month',readonly=True),
    'day': fields.char('Day', size=128, readonly=True),
    'partner_id':fields.many2one('res.partner', 'Partner', readonly=True),
    'product_id':fields.many2one('product.product', 'Product', readonly=True),
    'state': fields.selection([('draft', 'New'), ('paid', 'Closed'), ('done', 'Synchronized'), ('invoiced', 'Invoiced'), ('cancel', 'Cancelled')],
                              'Status'),
    'user_id':fields.many2one('res.users', 'Salesperson', readonly=True),
    'price_total':fields.float('Total Price', readonly=True),
    'total_discount':fields.float('Total Discount', readonly=True),
    'average_price': fields.float('Average Price', readonly=True,group_operator="avg"),
    'shop_id':fields.many2one('sale.shop', 'Shop', readonly=True),
    'company_id':fields.many2one('res.company', 'Company', readonly=True),
    'nbr':fields.integer('# of Lines', readonly=True),
    'product_qty':fields.integer('# of Qty', readonly=True),
    'journal_id': fields.many2one('account.journal', 'Journal'),
    'delay_validation': fields.integer('Delay Validation'),
}
_order = 'date desc'

def init(self, cr):
    tools.drop_view_if_exists(cr, 'report_pos_order')
    cr.execute("""
        create or replace view report_pos_order as (
            select
                min(l.id) as id,
                count(*) as nbr,
                to_date(to_char(s.date_order, 'dd-MM-YYYY'),'dd-MM-YYYY') as date,
                sum(l.qty * u.factor) as product_qty,
                sum(l.qty * l.price_unit) as price_total,
                sum((l.qty * l.price_unit) * (l.discount / 100)) as total_discount,
                (sum(l.qty*l.price_unit)/sum(l.qty * u.factor))::decimal(16,2) as average_price,
                sum(cast(to_char(date_trunc('day',s.date_order) - date_trunc('day',s.create_date),'DD') as int)) as delay_validation,
                to_char(s.date_order, 'YYYY') as year,
                to_char(s.date_order, 'MM') as month,
                to_char(s.date_order, 'YYYY-MM-DD') as day,
                s.partner_id as partner_id,
                s.state as state,
                s.user_id as user_id,
                s.shop_id as shop_id,
                s.company_id as company_id,
                s.sale_journal as journal_id,
                l.product_id as product_id
            from pos_order_line as l
                left join pos_order s on (s.id=l.order_id)
                left join product_template pt on (pt.id=l.product_id)
                left join product_uom u on (u.id=pt.uom_id)
            group by
                to_char(s.date_order, 'dd-MM-YYYY'),to_char(s.date_order, 'YYYY'),to_char(s.date_order, 'MM'),
                to_char(s.date_order, 'YYYY-MM-DD'), s.partner_id,s.state,
                s.user_id,s.shop_id,s.company_id,s.sale_journal,l.product_id,s.create_date
            having
                sum(l.qty * u.factor) != 0)""")

pos_order_report()

and the xml view for the same is

<?xml version="1.0" encoding="utf-8"?>
<openerp>
    <data>
    <record id="view_report_pos_order_tree" model="ir.ui.view">
        <field name="name">report.pos.order.tree</field>
        <field name="model">report.pos.order</field>
        <field name="arch" type="xml">
            <tree string="Point of Sale Analysis" create="false">
                <field name="date" invisible="1"/>
                <field name="user_id" invisible="1"/>
                <field name="year" invisible="1"/>
                <field name="day" invisible="1"/>
                <field name="month" invisible="1"/>
                <field name="partner_id" invisible="1"/>
                <field name="product_id" invisible="1"/>
                <field name="shop_id" invisible="1"/>
                <!--<field name="journal_id" invisible="1"/>-->
                <field name="company_id" invisible="1" groups="base.group_multi_company"/>
                <field name="nbr" sum="# of Lines"/>
                <field name="product_qty" sum="# of Qty"/>
                <field name="average_price" sum="Average Price"/>
                <field name="total_discount" sum="Total Discount"/>
                <field name="price_total" sum="Total Price"/>
                <field name="delay_validation"/>
                <!--<field name="state" invisible="1"/>-->
            </tree>
        </field>
    </record>

    <record id="view_report_pos_order_search" model="ir.ui.view">
        <field name="name">report.pos.order.search</field>
        <field name="model">report.pos.order</field>
        <field name="arch" type="xml">
            <search string="Point of Sale Analysis">
                <field name="date"/>
                <filter icon="terp-dolar" string="Invoiced" domain="[('state','=',('invoiced'))]"/>
                <filter icon="terp-dolar" string="Not Invoiced" domain="[('state','=',('paid'))]"/>
                <separator/>
                <filter icon="terp-go-year" string="Year" name="year" domain="[('year','=',time.strftime('%%Y'))]" help="POS ordered created during current year"/>
                <separator/>
                <filter icon="terp-go-today" string="Today" name="today" domain="[('date','=', time.strftime('%%Y-%%m-%%d'))]"
                     help="POS ordered created by today"/>
                <separator/>
                <filter icon="terp-personal" string="My Sales" help="My Sales" domain="[('user_id','=',uid)]"/>
                <field name="partner_id"/>
                <field name="user_id"/>
                <field name="partner_id"/>
                <field name="user_id"/>
                <group expand="1" string="Group By...">
                    <filter string="Salesperson" icon="terp-personal" name="User" context="{'group_by':'user_id'}"/>
                    <filter string="Customer" icon="terp-personal" context="{'group_by':'partner_id'}"/>
                    <filter string="Product" icon="terp-accessories-archiver" context="{'group_by':'product_id'}"/>
                    <filter string="Day" icon="terp-go-today" context="{'group_by':'day'}" help="Day of order date"/>
                    <filter string="Month" icon="terp-go-month" context="{'group_by':'month'}" help="Month of order date"/>
                    <filter string="Year" icon="terp-go-year" context="{'group_by':'year'}" help="Year of order date"/>
                </group>
            </search>
        </field>
    </record>

    <record id="action_report_pos_order_all" model="ir.actions.act_window">
        <field name="name">Orders Analysis</field>
        <field name="res_model">report.pos.order</field>
        <field name="view_type">form</field>
        <field name="view_mode">tree</field>
        <field name="search_view_id" ref="view_report_pos_order_search"/>
        <field name="context">{'search_default_year':1,'search_default_today':1,'group_by_no_leaf':1,'group_by':['product_id']}</field>
    </record>

    <menuitem action="action_report_pos_order_all" id="menu_report_pos_order_all" parent="menu_point_rep" sequence="3"/>
</data>
</openerp>

In the tree view you can find the fields for which sum is calculated as

           <field name="nbr" sum="# of Lines"/>
            <field name="product_qty" sum="# of Qty"/>
            <field name="average_price" sum="Average Price"/>
            <field name="total_discount" sum="Total Discount"/>
            <field name="price_total" sum="Total Price"/>

So if you have to add extra fields to the report, first you have to get the values for that field in the database view defined in init method and then you have to add the fields in xml view.