Maybe it's a points based system and how each coach views the player based on the criteria. You don't have to coach a player to know whether or not he's good at something.
In HS, when we watched and broke down film, you knew which DB you could pick on, you knew which LB to run away from, you knew which DE you didn't want to matchup against certain OTs. You don't think Tashard Choice knows which LB on the team gives his RBs the hardest time blocking on a blitz or is the hardest to run against in that LB's area? You don't think Marco Coleman knows which OL he can't successfully blitz against? You don't think coach Patenaude knows which player he can attack depending on the personnel?
Each coach recruits different positions in his recruiting territory, but I'm sure each coach can tell you what players he thinks might be a good fit for the offense/defense based on a set of criteria CGC has for each position.
In order to say a coach can't do something, you first have to know the criteria they use for grading. These coaches break down practice film all day, they watch their guys go against other players all day, and they witness how they do in drills. Remember, CGC is big on telling each player that they are also graded on intangibles such as effort, being a good teammate, etc. I'm sure that's part of the process.
Even though some can't fathom how coaches may be able to grade all the players, the point is, you don't know what criteria they're using, and you forget how much time these coaches go against all the players and what they see on film.