Tags and the hidden mass of the universe...
Exploring the forum features, I noticed that the "Tags" page contains tags which do not point to a visible post. The only explanation is there must be some hidden fora in which those tags appear; Which would also explain why the total amount of posts of all the registered users is way bigger than the amount of visible posts...
Many of the tags were added long before there were many posts at all in the forums.
As the forum grows I am sure that there will be more added as well.
As for the number of posts versus the number of visible posts, that simply comes down the the permissions.
Every forum has areas that are used by Admin/Moderators, etc; in the background.
Each tag was tested in such a forum, and that is why some presently don't go anywhere in the public forums.
We have done a lot of testing in the background, simply to get the forum ready for everyone and make sure it works as intended.