I've always wondered about this...

I've been a member of the BAEC since about 1995, and my account starts with 9xxx xxxx. My wife's new account is only a few years old (I can't remember what happened to her old account or why we signed her up again) and starts 4xxx xxxx. My children have been members for about five years now, and their numbers all start with 8xxx xxxx (and despite signing them up over several years, their sequences are pretty tight - less than 100 apart).

Any ideas?