Looking at the member list, only members who joined prior to 24 April 2006 are listed this way, all subsequent members are listed at varying times, so I assume it is due to the changeover. I notice that the first member who didn't join is me, as Shapley H. I entered a new member name because I couldn't log on under the old one, though I never posted under that name. The problem, if I recall correctly, was that the passwords became case sensitive after the changeover, and I had to enter the password differently than I was accustomed to doing. Once I did so, I was able to recover my old login.
Quod scripsi, scripsi.