PO line type matching basis

What would cause the matching basis on a line type to be "QUANTITY" when the order_type_lookup_code is "AMOUNT"?

I found this statement in document  1323973.1:  "The Value Basis column corresponds to ORDER_TYPE_LOOKUP_CODE and the Purchase Basis column corresponds to PURCHASE_BASIS. The MATCHING_BASIS is either 'QUANTITY' or 'AMOUNT', depending on the value of of ORDER_TYPE_LOOKUP_CODE.