Why does iExpense report_header_id sometimes not match invoice_num in AP_EXPENSE_REPORT_HEADERS_ALL?

According to the eTRM, invoice_num on table ap_expense_report_headers_all is: The expense report number, which is usually a combination of a customizable report prefix and the report header identifier.   OK, so our prefix is OIE and I would expect the numeric portion of that field to be the same as report_header_id.    In eTRM it does say "usually", so then, in what cases is it not that?

I ask because we have 7 expenses that have an odd looking invoice number that doesn't match the report_header_id.   We have something like this:

invoice_num:  OIE273223-2    report_header_id:  278371