Overhead costs in OPM Lot costing booked under PPV inplace of overhead absorption. Why?

We are process enabled organization with most of our manufacturing is Outsourced to vendors (OSP enabled). We currently use Lot cost as our costing method. We have very few items that have overhead costs associated with them. Currently everytime we manufacture the item with the overhead costs, there is a PPV generated though we were expecting ovehead absorption to be generated. Why is this happening? Can this be changed? Please advise.

Thanks,

Sowmya