It's kinda weird. Last time I checked the overall rating was 7.8 while all episodes except #2.6 were rated between 8.4 and 9.1.
shareI believe they have a weighted average which IMDB is vague about to prevent people from attempting to distort it.
I don't know everything. Neither does anyone else
(I know this q is from long ago.)
I've always assumed that the weighted average takes into account imdb's system of reviewer counts. The more reviews a user writes, and the number of helpful vs unhelpful votes they get on those reviews, applies a type of score to each user. Addition/correction of information may also be a factor.
Anyone with a more positive score has a bit more (weightier) influence on ratings throughout the site, certainly more than the casual voter who otherwise doesn't contribute at all.