While I know nothing about Clemson's situation, I'm going to guess it is due to very little attrition in the program over the last few years. Even though you can sign 25 a year you can only have 85 on scholarship at any given time. With that in mind, if you don't have people leaving your program and/or you replace every player that leaves with a walk-on that's awarded a scholly you can't continue to sign full classes. The numbers simply won't work.