Scheduling Agreement: Confirmed quantity is higher than Open quantity

Hello,

We person an Scheduling Agreement with Delivery Schedules functionality and I americium not capable to recognize definite behaviour of the system.

Cml. dlv. quantity: 52.626
Cml. released qty: 52.904

Schedule enactment with:

Order quantity: 500
Open quantity: 278 --> The strategy calculated the unfastened quantity from the cumulative delivered and cumulative received quantities. 
Confirmed quantity: 500

Why the strategy confimed 500 alternatively of conscionable 278?
If I privation to overwrite the confirmed quantity with 278 done the disposable power check, it does not instrumentality implicit 276, but lone 500:

Thank you successful advance!