PFF has their formula, and they really stick to it. Sometimes, a very good (but not great) player like Lavonte David with rack up bunch of plays by virtue of playing next to crap players. David is a very good player, perhaps even in the Top 50, but Top 5? I don't think so.
Then you look at a player like Earl Thomas, whose great play is not quite captured by the formula. They even acknowledge having to manually adjust their formula for Marshawn Lynch's "Beast Mode."