One other factor, great players miss less against players they know they will beat, so that sort of overrides the difficulty of the player. A long time ago, I realized I had become a good player when Jay Swanson started missing against me.
I'm just the opposite. I miss less when I'm playing somebody who plays equal to me or better.
If I'm playing somebody I know I can beat 90% of the time, I let my guard down and don't concentrate as much. Then, all it takes is a couple bad rolls and for them to get lucky and you are down. In short races you don't want to get behind.
The last time I played a tournament and I had to spot the guy some games on the wire and I started out "balls to the wall". I won the toss and then I broke and ran the first three games without him getting up to the table. Then lost the set 5-3 because I didn't pay attention to what was going on and the guy lucked a couple 9 balls in before I got my mind back to the game.
When I'm playing somebody who plays good, I usually play much better because I know I can't lose track of what's going on.