Well, in general, when people set up a webserver, they point http://
www.domain.com and http://domain.com to the same local directory on the webserver.
However, sometimes they forget to setup the "www", or they point the http://www.domain.com to (e.g.) /home/web/www and the other (http://domain.com) to /home/web/domain.com, which means that there are virtually two webservers running instead of one with an alias (DNS a-record). Means, they are treated as two seperate webs, which also means, one file, available on the 1st webserver, is not available on the other.
Bottom line is: System administrators are often lazy (no offense anyone). What you found is not really a tweak.
Tom