In Material determination, as per my understanding there is some standard program which is responsible for this behaviour. I donno much on that.
If you have noticed, the pricing of the main item in the order will be apearing as the pricing for the subitem in the invoice.
Even though we are delivering (PGI) the subitem, the sales order updates the main item as delivered (check the schedule lines in sales order after PGI is done).
Regards
Jobi