I'm pretty sure there is no sound explanation. If he wanted to pick the one that was most normal he should have just found the distribution that was quantitativ

I'm pretty sure there is no sound explanation. If he wanted to pick the one that was most normal he should have just found the distribution that was quantitatively most correlated with a normal distribution. The only rub in doing this would have been deciding on what an "optimal" value for the standard deviation in a 10 point rating system is, which would have been an interesting discussion. To me, this is the most important question of this whole approach. If really big standard deviations are fine, then Rotten Tomatoes uniform distribution might turn out to be the most "normal". But he seems to have totally glossed over this important fact, which is the real difference between IMDB and the other systems. With IMBD, the standard deviation of the scores is small compared to the range of possible scores. As a result, if a movie is rated 9.0, you know its gonna be pretty damn great, and a high 9 or 10 would suggest this is one of the greatest movies ever made by some distance. That's the kind of information you can't get on a rating system where 5% of movies get 5 stars.