Hi
At the time of GR mov 101 Inventory was received at standard price maintained in MMaster if the price control is maintained as "S" and same value has gone to your COGM
Now when you reversing the entry some other expenses has booked in your order and COGM cost has gone up...so when you reversing the whole GR qty the diff amount of COGM is posted to PRD a/c
If you give the details of your process and mat status like Price control etc ..it can be explained further.. But I think this will put some light on your query