No idea then - but it's happened at bag-drop at least twice. Just after I achieved status each time I think so I thought it was a thing they did to make you think it was really worth having achieved it.
I've gotten it once (HND to SFO), so it's not unheard of, it's just not official policy. 24 hours before the flight departs, the upgrade list gets transferred from the online waitlist to the airport, and check-in staff and gate agents have full control at that point (this is why UA doesn't let you use upgrades within a 24 hour window). Occasionally, you can get someone at the airport to put the upgrade through (though I've found it burns goodwill, so don't do it too often).