--------------------
My conclusion: The only pattern I can figure out is that the RSS feed only shows
new threads.
Only threads that are visible to unregistered users show up in the RSS feed it seems- so my test was useless to show that new threads are shown in the RSS feed.
I can only use
http://www.tfproject.org/tfp/showthread.php?t=134404 as proof that only new threads are visible in the RSS. When ngdawg created that thread I saw it in the feed, but the feed is not refreshed once new posts are created.