Your average cost moves on its own whenever stock comes in, and most of the time that is it working correctly. Here is how to tell which of the six it was.
A delivery arrived at a different price
The most common reason by far, and not a problem.
Your average cost blends everything you have bought. A delivery that cost more than what you were holding nudges the average up; one that cost less nudges it down.
Check: open the item and look at its recent receipts. If one is priced differently from the others, that is your answer. See What is average cost (MAC) and how is it calculated?.
The order had freight or duty on it
Extra costs on a purchase order are spread across the items on that order, so each unit ends up costing a little more than the price on the supplier's invoice.
Check: open the purchase order and look for extra costs. If there are any, a higher cost is correct.
Someone adjusted the stock
Adding stock through an adjustment changes your average cost, using whatever cost was entered on that adjustment. A wrong figure there goes straight into your average.
Check: look at the item's history for a recent adjustment, and check the cost on it.
A receipt was deleted
Deleting a receipt recalculates your cost from the deliveries that remain — it does not restore the number you had before. If some of that stock had already been sold, the new figure is genuinely different.
Something was received at zero cost
A purchase order created without a price comes in at $0.00 and drags your average down.
Check: look at the item's receipts and see whether any arrived at zero. This usually happens when an order was generated automatically and the supplier had no price set up.
Worth flagging to someone if you find it. It affects every sale of that item from here on, and the longer it sits the more of your margin figures are wrong.
It was bought in another currency
Costs on a foreign purchase order are converted to your own currency before being averaged in. If the rate on that order was not what you expected, the cost will not be either.
Check: open the purchase order and look at the currency and rate on it.
Still not adding up?
Send us these and we can trace it in one go:
the item's SKU code
the purchase order and receipt numbers involved
what you expected the cost to be, and what it shows