The reason is because the last post in that board was on the 2nd of July, and I suspect you've viewing only the last 30 days of threads in that forum. You can change on the forum page (there's a dropdown at the bottom of the page). Alternatively if you'd always like to see ALL threads then go here:
and change "Default Thread View:" to "Show all threads" rather than "Use forum Default" - which for a lot of the forums is 30 days. This then applies to all forums, and so no chance of missing a thread in future.
You're right, the total registered members isn't necessarily a good indication of how many people actually use the site on a regular basis. It's certainly true that members have registered, and then never been seen since - and dsmodi is also quite right there's nothing stopping someone reregistering under a different username AND email address.
As for unsubscribing we could easily add a facility to allow members to unsubscribe themselves, but that would leave the site prone to 13 yr-old hack3r deleting everyone! If members do wish to unregister and this has happened in the past, a quick email to me soons gets them deleted off the system, and their posts set to 'guest'.