This is how it is explained in the help section :
We now move on to the Player Game Plan page. There are 3 columns to adjust on this page, and each can be set to different levels depending on the defense you are playing against. We'll discuss each column individually, followed by adjustments that might be made depending on defense. The first column is the distribution for each player. This is generally a hot-button issue, so let's begin with exactly what distribution is. Your distribution numbers determine the percentage of possessions a certain player has a play run specifically for him. Your total team distribution cannot go higher than 100, but does not have to reach 100 if you do not wish. It's also important to remember that these are relative numbers. For instance, if you set all 5 of your starters distributions at 20, all of your starters would have an equal number of plays run for them while they were on the floor together. If you then replaced a starter with a player with 0 distribution, the 4 remaining starters would now have 25% of the plays run for them, while the player at 0 would not have any plays run for him. The distributions are constantly evolving as players come in and out of the game. Also, giving a player the highest distribution on your team does not guarantee he will take the most shots on your team. Distribution does not take into account shots from offensive rebounds and steals. Simply running a play for a particular player also does not guarantee he will shoot. The player may be fouled, pass out of a doubleteam, turn the ball over, or simply be defended too well to get a good shot. In the end, it is nearly impossible to have a game where your distribution matches perfectly with your shots attempted. Distribution simply determines who your team will attempt to get shots for. It is certainly no guarantee that certain players will get a certain amount of shots.