My
guess would be that the higher figure is for the total number of registrations, whereas the lower is the actual number of members. Before the move to the new software there were lots of spam accounts that got banned - but it wouldn't surprise me if they are still on the database, hence the discrepancy.
Could be wrong though. I'm sure Shaun will be along with the answer in a minute.
Edit: Still, either way, the forum is doing well.