Purchase Order Splits lines automatically

Sorry for the length of this question, but I am trying to figure out what is going on here. User has two open blanket orders that he releases Kanban orders from. The one that caused the issue had an original quantity of 700 on it.  He attempted to change the quantity on the purchase order (not the blanket) to 12 from 1. When he did this the order automatically splt the lines creating a line for 700 and a line for NEGATIVE 687. Why would it do this and do it on the purchase order and not the blanket. Any Ideas??