Camfantasy
Valar Morghulis
Perhaps they undersigned the previous year. Perhaps there were injuries or other holes to fill, other than drafted/graduated. Perhaps players know they want to sign with a school even knowing they aren't promised a spot, and will then juco it with the feather in their cap that a major school wanted them once and they might transfer after a year.
Just looking at numbers off the top isn't very telling.
Teams don't "under sign" by 10 recruits. The numbers aren't always the best indicator in a single year, but when teams do it continuously for over an elongated period of time....then it shows it's not just a few people not making the grades or them hoping for an outside chance at a spot on the team. All teams oversign to some degree. It is necessary to stay close to the scholarship cap. Signing 10 recruits more than you have room for every single year is not typical though, and there needs to be a rule set discouraging it IMO.