Chris,
One more thing I should mention is that if you use the "New Post" to look at all the active threads and have finished you use the "Mark Forums Read " under the "Forum Actions". This will clear the counter and start anew until you mark it read again.
When I've been away from the forum for a while the New Posts have accumulated for a while. After the last holidays it amounted to about 400 posts across the forums.
Peter.