The 25 limit.
Only 25 scholarship athletes can enter a class at the same time. So the class that starts in the Summer/Fall of 2012 can only have 25 athletes. When you see a school sign more than that some may be counted backwards, some may not be eligible. Unfortunately, some are simply told that there is not a spot. Some will greyshirt and delay enrollment until next year.
IMO, I don't think you should be allowed to sign more than 25 on signing day. But the NCAA rule currently says 28, so the system allows it. I would imagine if you see any school with more than 28, they have signed JUCO guys in the early period that will be backcounted towards the previous year's class.