Not sure whether the system is perfect. I was on an UA flight few weeks ago. Was on the DM list for an upgrade. Only 1 F seat left. My AC*SE (*G) status beat out several 1K & 1P and got me the last seat.

I didn't expect to get that last F so I just went ahead and boarded with my Y BP, it bounced at the gate and the GA told me my upgrade cleared and I got the last F seat.

(I was using a SWU and got on the DM list at least 15 hours before departure time when I checked in at HKG)