I would think so too, except I didn't enter it anywhere, so I'm not sure how the association is made. Oh well, no sense worrying about such mundane details.
The actual application form is on news.ycombinator.com, so it's got your cookies. Crafty, pg. Crafty. :)
To apply, you need to sign in using your HN username and password. If you were already signed in, you probably just didn't notice that the form already knew who you were.