WIP COMPLETION COST UNDER PAC

Hi，

Can anyone explain to me the pac calculation logic of wip completion？

For example, the job need $100 material and are all issued on 1st MAY. The job amount is 10 pcs.

I complete 10 pcs on 2nd MAY and then do WIP COMPLETION return of 1 pcs on 4th MAY.  The wip completion transaction on 2nd MAY is the final completion. So the WIP completion cost is $11.1

There will be another final completion of 1pcs next month.

I suppose the cost should be 10. Is this normal?