I'm a conspiracy theorist at heart, so I think PSA takes into account who's on the card (superstar/common), and who submitted the card for grading (auction house/regular guy), and hands out the grade accordingly. If you have a superstar, but you're a regular guy you can't get higher than a PSA 8. If you are an auction house, you automatically get a 1 to 2 point bump from whatever the regular guy would have gotten for the same card. The only way for a regular guy to get a PSA 10 is to submit a 1969 Rich Nye.
