That's not how IMDb rating works. It's just an aggregate based on the number of users voting on it. It's basically useless at distinguishing a stinker from a watchable weeknight movie from a masterpiece. Rotten Tomatoes attempts to address this imbalance with a weighted scheme, which usually works better for well-known movies.
I read the book first which made me really not like the movie. I think in a vacuum the movie is fine, but the heavy fictionalization to the point of preposterousness of something that was only one part of the book was just such a twisting. The book was about government waste, fraud, and abuse. The movie was about a wacky special forces guy who could apparently on some level actually use powers.