Why is cancelling partially received and partially billed requisition allowed when cancelling fully

When we try to cancel a requisition by using the “cancel requisition” button in iProcurement, we get a message “None of the lines on this requisition are eligible for cancellation” in cases where the PO was fully received but only partially invoiced (i.e. ordered = received > billed). But if we only partially receive the PO, and partially bill the received amount (i.e. ordered > received > billed), we do not get this message. Why is that?

Examples:

Ordered 4999, received 4999, billed 2499 ► ordered = received > billed ►“None of the lines on this requisition are eligible for cancellation.”

Ordered 4999, received 4000, billed 3000 ► ordered > received > billed ►cancel proceeds fine with no error or warning