Cleared upgrade, but somewhat not sure how...

So here's the story...

Last night I was on the late LAX-ATL flight, where I just missed the upgrade by one person, but then went mechanical just prior to departure. After deplaning and heading to the new gate where they were pulling a new plane in for, I noticed the delay was going to lead to a misconnect in ATL.

Fortunately, I noticed the MSP flight was boarding, so I mentioned to the GA to check to see if they could get me on that flight. She was able to, so I raced back to that gate where they were getting ready to shut the door in a few minutes.

What was interesting was that the boarding pass they had waiting for me at the gate was for somewhere in Y, but when they scanned it at the boarding door I was immediately moved up to 1C.

So I'm a bit confused here...

- Seeing the UG list for my ATL flight was near 40, surely all F seats would have been occupied on UGs.
- And even if there was a no show, they would have filled the seat by then I bet.

Not that I'm complaining here, but just wondering what roll of luck ended me up in 1C...