Hi Jamie,
The field AFPO-PGMNG is the quantity that you had planned in the planned order. Once the planned order is converted into a production order, the quantity that you see in CO03 is stored in the field: AFPO-PSMNG.
So you should compare PSMNG vs CO03. PGMNG could be compared vs MD13.
You can reproduce this in a few seconds: create a planned order for 4920 kg. Then convert the planned order and change the quantity to 4919.918 Kg and save. You will see what I mean.
Kind Regards,
Mariano