I wondered the same thing. Years ago, one of the recruiting services dinged your score if you lacked either a QB or a RB, but I didn't think that it was rivals.
I think it was rivals. at one point at tight end, fullback, or a punter weighed less than a quarterback or wide receiver.
I still do that to an extent but they just change the Stars. the best punter is probably still going to be a 2-star the best tight end of four Star if he's a pass-catcher.
I think they overrate the top 150 players. are those guys really worth six or eight 5.8 players? probably not but it's a recognition that the Tebows, Spikes and Harvins are ones that changed the game.
They misrank people too, like Braun, or Ethan White, that some of us including the Georgia and Florida coaching staff, saw as elite players.
they're far from perfect. they're just far better than 247 or other options.