Sorry the rabbit hole goes a bit deeper.

so there are 3 that changes the item "average cost"

A) when you create a PO and the item is not received yet ( new items)

C) when you receive the item. --> secondery use the PO price 

B) when you create a BILL for PO --> this one seem to the that quickbook use the most if created

still digging in but people who have advance inventory might have an option with "landed cost" 

if the average cost is faulty see if it is when you created a BILL and you did not add the correct value, 

if you are sure it is an error I'd delete the BILL +  Item receipt... it will then balance out