as mention before stamina might be the key.
But i like to give another XP explanation, Xp works afaik in two ways:
1. Decision making, an experience shooter know better when to take a shot or when it may be better to look for a better one during your attack. So your rookie might take an hard shot, or pass out of a good scoring oppurtunity more often then a experience player would do.
2. Late game performance, experience makes player clutch. i don't know how exactly it works, but a more experienced player performance better at the end of the game.
Don't know if the last shot in the quarter is affected two, but one is explained in the rules and 2 was said by Charles in the past.