The backorder processing ignores the quantity delivered, that is, it checks and confirms the entire order quantity thus having total confirmed quantity greater than ordered quantity.
How to reproduce:
Initially a sales order was created for quantity 10 and confirmed quantity 0. There was no stock.
EWM team added stock of 5 quantity and BOP ran and 5 was confirmed and same was replicated in ECC. It created delivery for this 5 quantity.
Again stock was added of 10 quantity in EWM and BOP ran and this time instead of confirming remaining 5, it just looked at ordered quantity
and confirmed entire 10 making total confirmed quantity of 15 (which is greater than ordered quantity 10)