There are a lot of factors here which could be:
1. Stamina. If the player has low stamina then his rating will be lower as he is unable to play well for long stretches.
2. Enthusiasm. Playing with higher enthusiasm, your players play a little better
3. Salary doesn't always tell the whole story. For all I know that 15k player could be something like 15 JS and 2 OD. Having one skill ahead of others inflates their salary extensively.
4. Position. Depending what position they play, ratings will be different. E.g. If you play a C at C he might get a 9 but playing him out of position at PG, and he will get a 3.5 if he can't shoot and pass.