Hi,
It's normal.
When you click on the login button, the system knows that you want to login and thus it checks the fields of the login form.
When you click on the register button, the system knows that you want to register and thus it checks the fields of the registration form.
When you click on the "next" button, all the page forms are submitted. If you fill something in the login form, it will try to log you in. If you fill something in the registration form, it will try to register you. And if you don't fill anything in both, then it will try to log you in as the login is the default action of the login view.
Now if you want to hide some of the buttons because you feel it is confusing, you can just use some simple CSS with the style display:none; in order to do so.