Qty doesn't update automatically if only the amount is modified during PO receipt for non stock

Hi All

In JDE 9.2.5 , I have observed the following 2 situations

For PO with stock line type S (Inv interface Y) , during PO receipt , if I modify the extended amount only , I receive an Extended Price Invalid error

For PO with non-stock line type B (Inv interface B with 3-way voucher match, Procurement Receipt Required turned ON in line type constants) , during PO receipt , if I modify the extended amount only ,JDE allows PO receipt. However , every time the full amount and qty is received

Questions

1. Why is the behavior different between stock and non stock as described above? I mean , why doesn't the system receive Extended Price Invalid error for line type B?