Your Vendor sends Bills without considering what Odoo says about the status of a Receipt in your system. 

They send a Bill when they believe they should be paid, and we won't prevent you from doing anything that may be operationally necessary to record your liabilities. 

Your Warehouse team may have forgotten to receive the products or made a mistake and received another Purchase Order.

If you create a new Bill and select the Purchase Order, 0 quantities will be shown as according to Odoo you have received none. This way your finance team knows something in the warehouse was not done properly, or that the Bill was sent before the goods arrived.