As everyone else said, the server has access to multiple IPs and it listening on all of them. not hard to do. I used to do it with a minecraft server had it listening on multiple ports, on multiple ips and multiple domains. And all of them could be listed seperatly. had the server listed 50 times, and all my members had to vote on all 50 listings to get a reward, But i made sure it was worth it.
You can do this with teamspeak as well, with iptables