Quote from BarbishkaGames
You guys do not understand, these are actual accounts which I will try to gain the screenshots now.
Type that link into the url area and you can see.
Sadly this is the only proof I have, but I can get more if you guys need. Please help the Minecraft community by finding this site!
First of all the screenshot isn't working, but the situation is quite simple. Eliminating the chances of them actually owning the accounts and logging into the same, non-cracked server, the only possible explanation is that they are using the /nick command on the server, and anyone can do this and change their usernames to anything they want, if they have permission to do so. If you want to do it yourself, type /nick <newusername>. If that doesn't work, either ask the server owner for permission, or find out if it's a different version of the "nick" command. This is all of course assuming that the server isn't in offline mode.