Well this site is still relatively new isn't it? One thing that I have noticed is that people tend to just kind of "float around the forums" as a guest for awhile before joining. I was a guest for awhile, just reading about stuff, until I realized I had to be a member to say stuff in the forums. Of course, I joined instantly, but to get to my point - I think many people use this site as a resource and just may not join because they come here for answers, not to give them
Of course, I'm sure there is a good percentage that is trying to do spam stuff too
guests are people who were linked from a search engine, those searching for answers and lurkers. Spammers need to join to spam and would be counted as a member once they join(before the final OK)