Why movemet 543 will effect the average moving price?

Dear all:

I have created a Subcontracting PO, then sent the RM A to vendor, and made movement type 541 in MIGO. After the processing in vendor, recepit the the FG B to our plant, and made movement type 101 & 543 in MIGO. The action should reverse RM A in our system, but I found the amount of reverse was not the same as WIP A before sending to vendor. And it has resulted in the variance in average moving price of the RM A. So I want to why is it. Thanks~