It is a complex of skills that determine the minutes distribution.
The starters are playing as long as they are better (in case of coach decision), the stamina will decide how fast they are loosing skills. Also the starters have some bonus over the bench (since you choose them as starters).
So if you choose your best players to sit on the bench and the margin is not too big with the bonus for starters they should be equal and may get the same amount of minutes.
So what count is:
Skills, Coach decision, stamina.
I think I read somewhere that they will improve it so you can have more options such as playing with the bench if leading by 20 points and such.