Basing the ratings off of the average rating can cause user ratings to go haywire. They can be too far off if a slew of poor reviewers rates it far from a good reviewer's ratings. Instead, the good reviewer would get a lower score while the poor reviewers will get a higher score. You would need a base with a true and perfect rating, but there is no such thing as a perfect review. The way it is has been working fine, granted that there are a few goofballs that might pop up here and there.