Online service

I am using an online service.
I log in with a username X then log off. Fine!
I then uninstall the sofware service and try to delete all of its roots elsewhere in the registry too.
Later, I install it again, and oddly enough, my username still appears on the log on screen awaiting for password input. Is there any file left I miss deleting in the registry > or could it be that the service's server's database stored my username and IP ? :(
By the way, what portion of the installed components in my computer stored the signature of datetime, my identities to match against the server ?

Help me understand it please. thank you.

That's absolutely impossible for us to tell you since we didn't write the software. But, remember that the registry is not the only place where data can be stored. Many apps use both the registry and local files.