You can over sign if you think a 4th year junior is going to graduate and move on because they never made it, or a guy like Bondi get beat out and decides to go kick for a small school his last year, or if you know someone is going to transfer, and you can plan for atleast one guy getting kicked off a year for being a dumbass.
We are not good at accounting for this. The difference is we will hold on to guys like Jay Guy and those types that never pan out and give them their 4 years to get a degree before we kick them to the curb. Alabama will cut you loose as soon as it becomes obvious your not going to cut it. That's the difference here, Bo does it the right way. It's not win at all cost, with him. He gives you the best opportunity to get your degree, even if you don't pan out, as long as you practice hard and keep out of trouble that is.