How to clearly distinguish a login page from a register page?

I recently conducted a series of user experience evaluation sessions for a prototype application that had been developed with the goal of capturing user experience bottlenecks and other issues.

Each test subject was placed in front of a computer with the application open on the login screen and a series of tasks for the user to complete. The first task required that the user register an account, which has to be achieved via the register screen naturally, but can easily be navigated to via the login screen.

Yet, in two-third of the sessions, the test subject began attempting to register their account via the login page. The subject either realised their error when the system returned an error as they pressed 'Login' or as they were navigating to the second form field.

Here are the respective forms:

Aesthetically, they are quite similar. The only differences are that the register form is, naturally, more lengthy and each form has a different title at the top.

My question is: is there a standardised way in which a login page can be distinguished from a register page, or is this confusion commonplace across different applications?