One factor that affects passing is shooting, and shooting affects passing. If a player has 13 Passing and has 10 JS/JR it might be better for him to pass it most of the time. However if he has 13 Passing and 15 JS/JR he is going to end up taking more shots and not passing the ball as much. Conversely if two players with similiar JS/JR the one with higher passing will end up taking less shots (but probably shooting a better percentage).