All inventory transactions in DBA are processed in real time.  In the case of PO receipts, real time processing enables received items to be immediately issued to jobs or picked for sales orders.    

The PO unit cost is applied to PO receipt transactions because in order for real time processing to work, each receipt must have a unit cost to update the item’s inventory cost.  The inventory cost is then applied to any job issue or sales order picking transactions.  

The supplier invoice cost is not used because there can be a considerable delay between the time of actual receipt and the time you receive the invoice.  In that intervening time the item may have already been used in multiple jobs or sales orders and its cost blended into other item costs.

Any discrepancy between the supplier invoice price entered in the PO Invoices screen and the associated receipt cost gets posted to Inventory Adjustments and makes no cost correction to past inventory transactions.  Minor cost adjustments of this nature are normal and expected.