What you need to remember are who the guys handing out these ratings are. I'm sure that a few pro football scouts started their careers as journalism majors, but i'd bet their aren't many. These guys basically write independent blogs. They basically rate the players by who has offered them, because they have no idea what they are looking at other than a highlight tape made by said players dad. Matt Elam was a 5 star. Why?... because he was huge and Alabama offered him. Saban probably offered him just on his size alone, but if he didn't drop 80 lbs. of bad weight by his RS Freshman year, he would have been transferring.
On the other hand TY Hilton torched us when he was playing at FIU... And no one else wanted him. How many stars do you think he had coming out of HS? that's right.. a whopping 2 stars. Why?? Because no one had given him a big offer.
Coaches evaluate talent and can see where those players fit in their program. Journalists look at stats, a highlight reel, and a list of offers. That's why you see underrated guys turn into monsters and overrated guys flame out. Anyway, that's my take on it...