Can't say about SFO <-> JFK, or if it's anything like LAX <-> JFK which is always oversold on flights I take. Might be because people know they have a good chance of getting an op-up? I'm happy to spend a few extra $ and buy 500 milers, which has worked well, especially from west to east. From east to west, I have been upgraded all 4 times this year, but once it didn't happen until boarding actually started.