Pro Football Focus inherits players once they enter the league, and does exhaustive statistical analysis. That's fine. Their numbers are impressive. In their related articles and analysis I think they miss the big picture quite often. For example, for Tannehill to boom into a great player he's going to have to overcome his atypical background. IMO, it's very foolish to project a severe upward tilt to a player who didn't start a season opener at quarterback in college until beyond his 23rd birthday, and who has never been great at any level. His yards per attempt the past 4 seasons are 7.0, 7.1, 6.8, 6.7. It's a significant sample size with no hint of brilliance. At some point that's difficult to ignore, or to happily adjust.
I would have far less problem projecting Tannehill to special status if he were terrific in college, let's say 8.0 or nearby. That's hardly an impossible hurdle. Every year in college football roughly 25 teams manage 8 yards per attempt or above.
I did great with California Chrome in the Derby and Preakness. There was no way I would have backed him in the Belmont. His pedigree didn't suggest a mile and a half, and the fact that he wasn't special as a 2 year old hinted that he wasn't Triple Crown caliber. All the legends like Secretariat, Seattle Slew and Affirmed were dominant at 2. The folks at Pro Football Focus would likely ignore those type of variables and strictly look at basic stuff like caliber of opponents.
Of course, I could be way off on Tannehill. Let's hope so. I always try to locate the path that will be correct more often than not, with as few variables and required research as possible. It's never designed to connect at 100%.