Minor fiddly question about Gmail

I have multiple Gmail accounts. When I start typing the username for my fannish account in the box on the login page (wherever www.gmail.com redirects to; it has a long address), a dropdown appears with that name in it. It only happens for my fannish username, not for my other Gmail accounts. I have Firefox set to never remember information, so I don't understand why it won't let go of my username; and if it's something it memorised back before I got Firefox's settings arranged to my preference, why is it only remembering one out of five?

I've tried messing around with NoScript settings, but it didn't help.