Lots of sold out games end up having seats, yeah. Gamers are a lazy and easily distracted lot, and GenCon's got (a) a lot of walking to do, and (b) a lot of other stuff going on. There's almost always a few folks who don't show up to almost any game -- too busy shopping, lunch takes longer than they thought, they can't find the place, they're just tired/beat from days of the convention, whatever -- so it's never a bad idea to show up to a game and wave a few generics around, to try and find a spot.