Stock sales being added to Purchases?

We understand that the sale of an inventory item is tracked through the Cost of Goods Sold account, but we have noticed that some sales of inventory items are being added to Purchases instead. We assumed this was a coding error and went through all items in our inventory and made sure they all linked to Cost of Goods Sold, not Stock Purchases. This fixed some of them but not all. We are wondering if this is some kind of historic issue, i.e. that the tracking adjustments only come into effect from the date the adjustment is made or if there is some other explanation?