Points per shot indicate the average quality of the shots taken by your players at the respective position. How many points you scored in a particular game or week will still be slightly randomized, so the difference will indicate how lucky/unlucky you were. Obviously, it is still likely that the points actually scored are close to the PP100 figure.
"I don't know half of you half as well as I should like; and I like less than half of you half as well as you deserve."