Actual Subcontract Costs

When viewing job costs, I am perfectly OK with the estimated subcontract cost. However, I can't figure out why our actual subcontract costs are so much higher. I can add up all the invoices on the subcontract PO, and the actual subcontract cost on the job will still be higher than that. Is there something else that gets factored into actual subcontract cost that I am missing? Thanks!

Adam Whipp