What is the difference between logging in to the router and logging in to the router using TP Link ID?

In the below screenshot, my login fails if I enter my admin password and click 'Login', but if I click on 'Login with TP Link ID' the screen changes and I enter my email address and my admin password and it accepts this login. I don't understand what the difference is?

Also, I can't change the router password if I use 'Administration' under 'System Tools' under the 'Advanced' setup tab, but, under the 'Basic' setup tab, if I click on my user name it takes me to a screen that displays my username and hidden password, if I then click on the box with a pencil in it, it let's me change my TP Link ID password with no problem. Again, not understanding this, is this the router password or not?

Thanks,

RouterNut