Material OH absorption for Work Orders with Buy parts

Hello,

We are on 12.2.3

If you have a WO with an assembly that is a buy part(it has material and material overhead costs),upon completing it to invenotry we notice that the accounting is as follows:

Dr:Inventory for the total cost of the item

Cr:WIP Valuation for Material cost

Cr:Overhead abosrption a/c for Material OH a/c

Why does the system Cr overhead absorption and not the WIP valuation in this case?

Thanks

Rahul