Im looking for an explanation as to why sometimes the upgrades get processed ahead of time, and then other times it doesn't happen until the gate, even when there is many seats open in FC.

The reason I ask is because it has happened to me many times, but most recently this weekend. I had a round trip flight EWR to POP. Flying to POP I received my upgrade ahead of time as there was 8 seats open in First. Coming back I did not receive the upgrade until I got to the checkin counter, even though there was 6 seats open in First. What is the reasoning behind this? I called CO for an explanation and was told there is none and she can't explain why this happens lol

FYI I am a CO Onepass Elite always traveling on original CO planes/routes out of EWR, never United flights so I don't think that has anything to do with it as in the systems not being fully integrated.