I think about that concept all the time and I'm a bit on the fence!
On one hand, 75 for a 'perfect' performance on an average game doesn't seem fair, but on the other hand, it does differentiate between the referees who then go on to have an excellent performance on an extremely challenging game.
The luck of the draw I suppose - 3 easy games = 3 average assessments, whereas someone who gets 3 challenging games is going to come out on top, even if the referee who had the average games would have handled the challenging games as well, or even better.