your post reminds me on a kids game " I spy with my little eye" ..a delivery .. with a different quantity , now tell me from why it is wrong.
What IDoc do you use?
what segment and field has the quantity you are talking about?
in what transaction are you looking at what field? tell the transaction and the technical field name
Has it ever worked in the past or is this your first try?
Is it just standard SAP inbound function or is it enhanced with own coding?