OS X assumes hostname of other network computer: how to prevent this?
This issue has been going on for years, and there is no way to "fix" it on the Mac.

What happens is the machine gets an address from DHCP, and then performs a reverse DNS lookup to see if there is a hostname associated with the IP in DNS.

If you are on a Windows network, all of the Windows machines are automatically registered in DNS when they connect to the Domain.

I went on and on with the techs at Apple about this... There is little that you can do at the level of your Mac.  It becomes an issue for your network administrator, and since there is nothing really wrong, he/she is likely to leave things as they are.