It could be because phpbb's using AdSense, which sends a spider out every time a user visits the page. Google claims that it's a separate craql but my own SEO research has led me to believe that AdSense publishers are indexed more rapidly in the search engine as well. But I'm starting to doubt it, Google recently has changed the way they spider and are updating on a much shorter schedule than in the past.
Google is doing the google dance, and it's a big dance right now and A2K results are coming in and out.
But it's important to note that you won't find all of phpbb's topics indexed and you won't find every single topic here indexed in Google.
But the limitations for how much Google will index have to do with limitations on Google's end. Page rank is one factor they use for this.
To search for topics from this forum that are indexed in Google use this search query