If a movie belongs to a strict franchise, it's quite clear that most people seeing and rating it will be fans of the respective franchise. That is why action and comic book movies will generally get a higher rating. It's like the (mentioned above) TV series example. Movies for the general public with sufficient votes though will usually have the fairest score. Newer movies are usually rated by 'positive' critics at first for some reason, but I have seen the exact opposite for a few titles, in which the initial score was lower than the final score. It all depends on the vibe generated by the movie.
Yeah obviously there's a bit of "fanboy" voting going on. But still, it's a bit of an issue for credibility when you have Avengers at 8,2; Racing Bull at 8,3 and Taxi Driver at 8,4