Ask HN: Why is GitHub's “Landing page” so weird?

It's been bugging me for a while. When I go to github.com and I'm not signed in I get my username and password auto-filled into the "Sign up for Github" form with my username in the email text box, my password in the password box and the username field blank. This is strange enough as most websites I can think of present a login form with a "Sign up" link for new users, but I actually have to scroll up for the sign-in link.

I don't really care, but today I had a few minutes and figured I'd post a question on HN just because I'm genuinely curious why someone would design a login experience this way.