I need help with AP_MATCH process

I'm looking for help understanding the AP matching process.  We load invoice data and PO data thru sqr interfaces.  We also have match rules written to match on the INV_ITEM_ID field.  Our issue is this, during the AP_MATCH process, invoice rows are correctly matched to PO rows based using that match rule.  If we have 8 rows on the invoice side and 10 rows on the PO side.  The program will correctly match those 8 invoice rows.  But then when it comes time to actually mark the PO rows as matched, the program will insert invoice line_nbr's 1-8 into a