I was flying ORD-LGA very recently and on the bubble for the UPG. I ended up missing it because they took two FC stand by passengers. Based on the amount of seats available and my position on the UPG list I should have gotten the UPG, but since they took the two paid FC stand bys I was out of luck. GA confirmed this because I went to check on my chances and she said there are two people not on the UPG list that will make you miss the UPG.