I didn't list a year when I created my journal, because I wanted my birthday to be available (e.g. on the portal page) to my friends, but I didn't want to publish my birth date to the whole internet alongside my real name. Many older users do this, so we're uncounted in these stats (or incorrectly counted, maybe?).

Just in the last few days LJ fixed this; you can now enter your birth date but control what subset of it is published.