The short answer is: it's intentional. I prefer it as it keeps URLs shorter, and I don't like typing extra "W"s for no reason.
The long version is because "www." is not part of the site name. The "www." subdomain started being used because back in the dawn of the web, many large companies and organizations need to run the webserver on a different address/machine than other services like ftp or telnet, and there was only one website for the whole company or university. Here though, there's lots of sites, so we use subdomains to separate different user sites... so the webserver can read the first part of the server name, and know whose site it should go to. When you put an unknown (or, more commonly, expired/deleted) username in there, the webserver redirects to xepher.net itself. Most websites do that for unknown pages. Since it doesn't know any user called "
www.username" it redirects. You can try any gibberish in front of xepher.net and it'll do the same thing.
http://pekdhdkawimvo.xepher.net/ , for example, will go to the front page as well.