The variance comes because we get lots (sometimes hundreds) of people who sign up for an account and never activate or activate the account and never re-visit the site for more than a year. I periodically purge these dormant accounts. I think many of these folks find us through a web search, sign up, and then figure out that SOHC/4 has nothing to do with the bikes they're interested in. You don't need to create an account to read most of the forums; just to post.
A somewhat different, though related issue is the SOHC/4 Owners Club registration, now standing at 3434 members, which never gets purged. Members retain their number forever. Some may decide that they don't want to be listed in the directory, but I still keep old bike and member information on file.
--Glenn