IMDb miscalculates ratings.

We take all the individual ratings cast by IMDb registered users and use them to calculate a single rating. We don't use the arithmetic mean (i.e. the sum of all votes divided by the number of votes), although we do display the mean and average votes on the votes breakdown page; instead the rating displayed on a title's page is a weighted average. To display the detailed votes breakdown, click the number of votes located directly below the average IMDb user rating. For an example, see the User rating breakdown for Inside Out.

The IMDb weighted average does not change upon receipt of each new vote, but instead is updated numerous times per day.

A TV series rating is not the weighted average of the ratings of individual episodes. Instead, customers vote separately for the rating of the series as a whole via each title's series page.