Recruiting rankings are the ranker's guess as to the player's potential
and current standing amongst his peers.
Rankings do not mean a guaranteed level of success. A player may develop to be better or worse than their ranking was coming out of high school for a variety of reasons. And rankings are educated guesses, not exact science. With that said I would say that in general the ranking services do a reasonable job, yes there is room for argument, and yes they get things wrong, but they also get a lot right.
As for Slade going to ODU, my guess is there was not a lot of interest from big time schools including VT.