Behavior seems random and unconnected as I did add a new RSS feed starting with "B" and Mail placed it to the bottom of the RSS feed list. Then, I renamed it by deleting a word at the end of the feed name so it kept its name starting with "B", and Mail sorted it rightly. This trick didn't work with other feeds though, hence the randomness I was talking about.
Also, the actual url for the feed doesn't seem to matter either, hence the 'unconnected.'
Mail Version 5.2 OS X Lion.