I would think one of Jon's IT/Dev guys could explain how the site remembers that you've already signed up. Is it just a cookie stored on the local machine? Or does it store it as an attribute of your account, meaning you'd have to be logged in for the site to know that you'd already signed up.
if it's cookie-based, and your browser is deleting its cookies every time you close out, then the site assumes you've never visited HN.com before.