tactics is one. if you play say an outside tactic, then the one with the better outside shot will get the ball more often because he has the best shot available.
Defenders, if the person defending your SG is a scrub, and the person defending your SF is an allstar, chances are, the SG will take more shots becuase they will be better. THe game engine tries to find the best shot possible, and that is how some player shoot better than others.
Perhaps even the passing skill is involved, if player 1 can pass well, but player 2 cant, then player 1 will get solid passes to player 2 and player 2 will attempt more shots because of how good the shot selection may be. and vice versa, if player 2 gets it to player 1 with a bad pass, then it wil lbe a worse shot attempt, and palyer 1 probably wouldn't take it.
Hope that helped
anyone may feel free to correct me to clear up any confusion.