I dont know that much about making web pages and stuff, but i am fairly sure i know what you are referring to and i believe it is because the site owns both adresses or something like that. Similar to typing 'www.yahoo.com.au' will redirect you to 'http://au.yahoo.com.' I have always asumed this thing is becuase the site (yahoo in this case) has made the first adress automatically redirect to the second site as it is the mainstream site.