According to something I read many many moons ago (source?) IMs tend to have full-time jobs or careers away from the chessboard and are, by and large, not full-time chess professionals. Some GMs also have other "jobs" or careers for instance Vidmar and Botvinnik were electrical engineers, and Tarrasach was a medical doctor but by and large GMs more often tend to be full-time chess professionals with fewer "distractions" to keep them from reaching their full potential.
See IM Silman's remarks about Deep Thinking and the Differences Between Titled Players for more details by someone who probably has a little more expertise on this subject than most of us
http://www.chess.com/article/view/deep-thinking-and-the-differences-between-titled-players
It wasn't a good idea to use Carlsen's name to explain some truth. My mistake. I take back the name. And I'm not in a position to tell who could/should be at the top of the chess world. But still I support my claim about IM's and GM's playing power can't be comparable by current conditions.