As far as critics vs fan scores - and I say this as a critic (well when it comes to games anyway), a lot of critics can tend to have an agenda, focus more on the artistic nuances vs entertainment value, and/or could somewhat have biases thanks to Disney accommodating them. Not going full-on conspiratorial and saying these guys are paid under the table or something, but it could even just be more indirect by getting free screenings and thus looking at the film more favorably, or simply not wanting to piss off Disney execs and "rock the boat" too much. I know it my experience, while I strive to be objective as possible, this can be a tougher feat when tossed a free copy of a game.
With user scores, while many of them can also certainly come with their own agendas, there's at least a far greater pool of people to pull from so the cumulative average tends to be a bit more in line with the actual quality in terms of accuracy. The users tend to speak a bit more from the heart and have no reason to go out of their way to provide a review so you get the sense that they're (typically) more authentic. They aren't being paid, they don't need to bring traffic to their site/blog, etc. Of course - you DO also get the issue with fanboys of other consoles purposely trying to sabotage reviews (Sony vs Nintendo and vice-versa), though I think this is far more an issue in gaming than in films unless maybe we're talking DC vs Marvel or something.
That's not to discredit reviewers - I mean hey, they're getting paid usually to do what they do, so they obviously know what they're doing to SOME degree. But I think some forget that critics are just fans too (and sometimes maybe not EVEN fans), just fans that tend to be better writers than the average fan :P
