Cpanel doesn't need a username to login, if I give the correct domain name as the cpanel login URL

ie, for the

domain --> test.com
user --> test
password --> testpass

I can login to cpanel with the following.

Cpanel Login URL --> http://test.com:2082
Password --> testpass

I don't need to give the username here. Is this a bug?
I tried it on all our servers, and it's the same on all servers.
I even tried it on a server with a different host.