Indeed.. there is something not right. This particular post is also in there.
I just punched in the URL of the subforums (
http://www.guitarforbeginners.com/fo...splay.php?f=71) in Liferea (linux feed reader) and it autodetects this feed:
http://www.guitarforbeginners.com/fo...l.php?type=RSS
Other forums give the same result so that leads me to conclude there is only one feed that covers the whole site.