Take two 4 star recruits at the same position, both of whom are being recruited by BYU and numerous other P5 programs. Recruit A chooses BYU, recruit B goes to a higher ranked P5 school. Although both are 4 star recruits, there's a lot of reason to think player B is the better player. Player A knows more about his abilities and work ethic than the rating system. Same with Player B. Assuming both players are trying to maximize their playing time and visibility, Player A must think that he has a better chance of playing at BYU. Player B must have more confidence in his ability to compete at a program where the depth chart is more stacked. So the 4 star recruits who choose BYU over other top programs reveal something about themselves. Same with the recruits who go to top ranked programs.

Obviously there's tons of confounding factors (honor code, personal values, future goals), but I think over the long run this effect is probably real. It's a theory, but it makes sense to me that the guys who could go anywhere but choose BYU might know something the rest of us don't about their true abilities.