I guess their could be a number of reasons.

A, they could all be guest users brouzing the site wondering if they want to sign up

B, they could be people who want to post, and want to say hi, but not sure what to say or how to introduce themselves

or.. maybe they are just that

lurking on the forums so they can get the information they need, then leave

their's a lot of info here, I suppose it's quite possible that a user can just find what they want without posting a question themselves.

another possibility is that these people want to post and comunicate, but arn't really good with technology, and don't know how to begin