Jay, please clarify which RSS feed.  The one from the bottom left corner of a forum thread view or the one from the module action?

The one the module exposes is based on what is indexed by the portal search engine.  This is a feed for all forums and as developers we have a limited amount of control of what enters here.  When the search was developed, it did not take into account some of the more complex scenarios like the forum exposes.  This would have been very difficult to account for when creating the search engine since it needs to handle this in a very 'generic' manner.

The button in the bottom left corner exposes a specific forum to RSS, unless it is private.  This is built into the module and will expose everything based on forumid.