Purchase price variance measures how far actual purchase prices differ from expected ones. In a standard costing system it compares the invoice to a standard cost; in other systems it captures the part of a price difference that cannot be pushed onto inventory still on hand.
A distributor might sell part of a shipment before the supplier's invoice arrives at a higher price. The cost of what was already sold cannot be changed after the fact, so the difference lands in purchase price variance.
In APFoodz price differences found when a supplier bill is matched are accounted for automatically.