Purchase order control status closed

Hi,

As per my understanding once we prepared AP invoice matching against PO then the PO control status will change to closed automatically, but i notice some PO status shows closed and some shows open. I want to understand when exactly it get closed automatically. Its an internal audit query which i need to answer, please advice.

In the attachment most the cases when PO amount fully matched with invoice then the status getting closed automatically, but some cases its open highlighted in yellow color, please advise.

Regards,

Javeed