There's another possible reason:
Guests can't see the attachments in the posts. You have to be a member to be able to do that.
I think the same goes for the member albums, and maybe some links, too.
When I found the forum, that was my reason to join. The discussion and sympathies about Tigger's truck crash led me to return and participate. Now it would be hard for me to live without it.
Some new members may have joined to see some attachments to posts, but didn't want to participate. Some of them may have come occasionally, some may have never returned.
In the last months, some wannabe spammers may have joined to use the forum to lure members somewhere else by PMs, just to find they can't.
