It's cos you use the www on the front of the site and LJ is linking to the non-www version, plain ol cellar.org without the
www. (which is the preferred way BTW)
When you log in, the system starts a cookie or session for one but not the other, and so it figures you are a new user if you switch between.
I could have addressed this properly at any time, but half the people would have to re-log-in the next day. So what I figure is that I'll force the non-www version whenever we switch to the next version of the vBulletin software, and handle all the big change at once.
(Which we will, eventually... it's in beta now, and it's VERY good from what I've seen. Loads of forumy goodness.)