Subcontract Cost shows negative on Prod. Order Statistics

Dear all,

I am testing the flow of overhead and variance on production. So I set up for all work centre, raw material and finished goods with overhead rate. On the work centre of a subcontractor, I set to use "Specific Unit Cost" and 20LCY of overhead rate. Then I check on the statistics window of the production order, and found that it shows -20 for subcontract cost, 20.18 for capacity overhead, where 0.18 is the actual cap. overhead I defined.

I then look up in prod. order routing line and search for the subcontracting line. It shows -20 for direct unit cost.

Can anyone explain why this figure is showed up in negative in this case? I am using 5.0 SP1 W1. Thank you.