I have some categories in the forum, the first one is "Announcements". There is only one Post "welcome to the forum", which dates from 2008.
Now, searching to "widget forum" gives my forum on first rank, but my forum is listed like: 1 Post - 1 Author - Last post: 2. Apr. 2008
This makes my forum seem to be completly outdated.
So I moved the categorie to the end, which makes new posts appear on the top (new posts are written every day). This was 8 weeks ago, the Google-Cache has got the new structure, but in the SERPs Google is still claiming: 1 Post - 1 Author - Last post: 2. Apr. 2008
What can I do about it? And still there is the problem: Why does Google think there is only 1 post and 1 author?
What jim said. Google has written seemingly custom regular expressions for forums across the web. A minor change in your code can cause it to break and render the 1 post, 1 author thing (you'd think they'd have a filter in their code to not display when it's something as uninteresting as 1 post, 1 author, which to me means they actually WANT people to know/think your forum is stale and uninteresting and take it into account when clicking through results).
There is no recourse besides posting in the Google support forums and hoping someone pays attention, and no documentation about the feature exists.