How routers manage to login web with blanket username/password ?

During the setup of a router recently purchased to replace my old faulty one , I stumbled on my own misconception.
The manufacturer's instruction was to chose PPPoE menu on the Internet connection type , which prompted fields to type username and password.
It was recommended for my DSL provider to type "client@client" in the username field and "client" as password.
I gave it a try and surprisingly got a connection (lan of my desktop).
A couple of years ago as I configured my old router , I thought that I should type into the equivalent fields of the TP-LINK interface , my DSL authentication email and respective password. On that time I did it and it worked.
I thought that the router employed such info to dial through the modem and to log in the server of my provider.
Then on the new router interface I typed my authentication email and password , it worked as I expected it would.
Minutes ago , I discovered that I had mistyped the password : I missed one letter and even so I got the connection (and not an error message ). That means anyone who has a DSL modem and a router can use my phone line and to log in.
The same does not happen If I plug the modem direct to the LAN port of the desktop , in this case the authentication info is required.
How the router manages to dial without the correct authentication info or with a generic username/password ?
Thanks