Quantity on Hand and Forecasted quantity of product X is 10 units.

I create a sales order line with 5 of product X and confirm the sale. Forecasted Quantity of product X is now 5.

I decrease the quantity of product X on the sales order line to 2. Forecasted Quantity of product X is still 5 - I would expect it to be 8.

I change the quantity of product X on the sales order line to 7. Forecasted Quantity of product X is now 3.

Is there a reason that reducing the quantity sold does not increase the forecasted quantity?

Am I missing a setting somewhere?

Does anyone have a suggestion how I can make the forecasted quantity reflect the reduction?

Odoo 9.0c-20160915 (Community Edition).