I think the whole rating thing is subjective. I mean take for example the "C" player. In my mind, a "C" player is "your average player". In all my years playing pool and owning a poolroom, seeing hundreds, perhaps thousands of pool players, a "C" player is lucky to run 8 balls in a wide open rack. That, to me is your average pool player.
Better than average is ( at least in my mind) a "B" player. A "B" player, being someone who can run through a full rack of balls and 40 percent of the time get on a break shot and get into the next rack.
Now that has to be better than average.
I think an "A" player is someone who can (with regularity) get through 2 or 3 racks.
Then ( again IMO) the rating goes on to AA AAA and Pro.
The point is I'm basing the lettering off of what an average pool player actually is, and the average pool player is not running multiple racks or even one full rack consistently.
In fact if we really did a scientific study, I'd be willing to bet that the overwhelming "average player" may fall more closely into the "D" player category. :shrug:
For as many years as I've loved this game of ours, I can recall at least a half dozen times we've been to parties or social events where the home owner has a pool table in their house. The conversation comes up that they are always playing and love the game. A few other guests agree that they also love to play and when we all finally converge in the basement around the table, I'd be comfortable giving them 3 to 1 on the money my wife would kick their ass, and I'd be stealing if I played them one handed. (and I suck one handed.)

So, IMO there is your average player.