Howdy!
The "amount ordered" on your OP will not decrease when You make receipt. There are quite a few amount fields in PO details table.. Amount ordered, amount received etc.
When you make a receipt, it updates "Amunt open" and "amount received" columns to reflect your receipt/receipted amount but "amount ordered" will always be the order quantity x unit purchase price
You can see this if you open databrowser and look at table F43199 which normally captures all the changes to the purchase order line (depending on setup on order activity rules) and this will help you see visually how you PO detail line was updated from the reeipt
The processing option you mention is used when you take row/exit to "Receipts inquiry" but has no impact on the actual receipt process which is done in P4312 application
Hope this helps
Aarto