I would venture to say that the post counter was dynamic and calculating each poster's post count was computationally expensive and significantly slowed down page loading.
The post counter is also inaccurate; the user_posts integer in the php database (MySQL I suspect) simply ticks up on each user post. There isn't a function to -1 post from a user if a post has been deleted.
So if a user posts 30 times to a thread that gets deleted, those 30 posts are still counted, even though they no longer exist.
So no, it's not dynamic as such, it's a static value that is pulled from the db. The only query made to the db is "What's this user's user_post count?" not "Search for all posts by this user and return a value equal to the number of posts" (which would be much more accurate but would chug the site horribly).