most of the reviews are from professional reviewers?
To further clarify -- RottenTomatoes actually has two lists of reviewers. There's a general list, which is what the score on the main page (and everywhere else AFAIK) is based on, and then there's a list of "trusted" reviewers. I dunno what makes you a trusted reviewer, but I'm sure you could find out. It's a smaller list and generally has a (slightly) different score.
But, AFAIK, neither list includes reviews from random Internet bloggers, posters, voters, etc. (unlike IMDB). James Bernadelli is a "trusted" reviewer even though he's not professionally employed as a movie critic, but he's been posting reviews on Usenet for over a decade now.