imdb's rarings often bewilder me. Shit films get good ratings. Great fims get bad ratings. But I don't think imdb's ratings necessarily accurately represent the views of the public out there on the street.
And yeah ROTS getting a 7.9 is ridiculous. The critics were dumb on that one too -too many of them thought the film was good. Though you did get some more accurate reviews, such as in Rolling Stone. ROTS was a masterpiece of making little out of much, making crap out of good material, and it's so bad in so many ways that I can only come to the conclusion that a lot of effort was put into making it so bad. For sheer lameness it has most movies beat. Think of some of the stuff in it -Yoda vs Sidious, the Sidious overacting, Annie vs Kenobi on Mustardfart, moronic lame Padme, Annie coming off like he goes dark because he's too dumb to know better, General Grievous, awful romance bits between Annie and Padme, Yoda strking macho poses, stupid annoying jedi get wiped out and it's supposed to be tragic and you go "It's about time, the wankers.", not to mention "You underestimate my POWERRR!!", "I hate you!!", "Noooooooo!", etc.
I think one of the reasons the film got better reviews was the political posturing that was in it. That made it to some degree "clever" in the eyes of some critics. Lucas got points for being politically "relevant". Another thing is I think some people took the big epic tragic storyline at face value. Yes there's this big epic tragic thing in it, but it has no emotional depth and thus doesn't really count for anything in the end result. But I think some people figured that if it had this epic-tragic thing then it must have the emotional resonance that must go with that, and thus they read into the film feeling and depth that wasn't actually there. Personally I think that big epic tragic element makes the film worse, because it means there was more in the material that could be made into something good and the fact that Lucas made so little of it is even worse than with a storyline of less substance like AOTC's.
Still, when all's said and done, the prequels as a whole are looked down on as failures.