I always use /join to go to another server quickly. Problem is when I go to a server which is a temporary server (player made), I go directly to a new room which I am the administrator. Is there something wrong?

you spelt the name of the server wrong

Which makes you create a new server [and your admin of it].

No I meant is for example I'm going to join server 15 where there are some people playing. But when I type /join 15, It goes into a server where I actually made the server.

You have to type /join <Server Name> not number.

I have the same problem as basher. /join 15 and manually joining the #15 server by doubleclicking it makes me end up in different servers.

/join goes to names. These names are not related to the number.