Approval Processing - E1 Procurement

I have a 2 lines on a PO

Line1 is 100 $ is approved

Line2 is 100 $ is approved but later user decides to cancel this line

Question why does the order go for re approval , the order total is reduced but is this the reason ? why would reduction in value of a order trigger an re apptoval.

**( cancel date is not part of user approval critical fields ), can some clarify on this scenario, why is order going for re approval?

thanks,