Keith,
not sure if something is happening at the delivery level (probably some corrections based on the way the material is picked which updates the delivery quantity with some exit) , can you check if you see any change logs in Delivery level, if you dont have the authorization to check the changes in the delivery level, check executing WSCDSHOW program for that delivery number & let us know.
This seems to be a peculiar issue, as Lakshmipathi pointed out this might because of some user exit.
Regards
Ilango