In OPM Lot Costing, Batch WIP Completion Returns shows incorrect lot cost

Dear Gurus,

I am working on Lot costing, where in i am creating a production batch.

Then, consuming the ingradient and yielding to a existing lot. Further I would be doing Completing the batch.

Next, doing a reopening the batch -> WIP and completing again.

This means, I am doing WIP Completion and WIP Completion Return and again WIP Completion.

After I run the lotcost process, lot cost for WIP Completion for the second time is not the same as first one.

If any one has come across or have a probable view , I appreciate your help.

Thanks

Raghu