Purchasing - Line splits when quantity is increased

Hello,  I am looking for suggestions on what may be causing the following issue:Scenario:A purchase order is created from a requisition (original order number exists), could be for a single line or multiple lines, does not seem to matter.  After the purchase order is transmitted and an ackowledgement received from the supplier, the quantity needs to be adjusted.  If the quantity is increased, the amount of the increase is split to a separate line on the PO.  This does not happen for all part numbers, just some.   This seems to be part number specific, because I can have two different