I'm guessing Google limits the number of documents it crawls under a given URL (eg, openacs.org/bboard in this case).
Maybe it does this as a precaution against "infinite" URLs. For instance, imagine crawling through our very own calendar application. If you followed all the links, it would never end since you would be crawling through all the years (2003, 2004, ...)
That's my theory anyways. =)