If my player (say SG) has a matchup rating of 100, that would mean that I would expect him to shoot 50%. This is a weighted average of my multiple players offense against his multiple players defense and is calculated after the game so that the minute distribution is taken into account. It includes only two point shots and not three but that should be a relatively small effect.
In that case, I'm lucky if my SG shoots over 50% and unlucky if the SG shoots under 50%. It may not be so simple because of the weighting over different players, but just as a first order assumption.
Is all of that correct?