some are last posts and some are member profiles which may not have alot of info as indicated elsewhere. But some are showthreads and archive'd posts. So what gives?
There are two stages of page index within google. Partially indexed and fully indexed. Fuly indexed have a cache, and show page title snippet etc. Partially indexed pages (PIP's) are pages that google knows exist as they have found links to it, but have not yet indexed and cached it. Of late google has been dropping fully indexed pages that might appear to be duplicate content, down to PIP, then dumping them.