The profootballreference FanEloRater is not any kind of "SABR" metric, it's a stupid fan generated ranking. Anybody can vote, just click the vote link, and you will be presented with a pair of players. Pick the one you think is "better", and they are given points while the "loser" is deducted points. That's how players move up and down the list.
It started on the baseballreference site, where it works much better because players are more easily comparable. How the hell are you supposed to compare a QB to LT or a K to a SS? It's stupid, it doesn't work, and the rankings are wacky.