HOLD ON.
There's a very simple explanation for this. Within the schema of 'off topic' exist *every single topic that could possibly exist that isn't covered by another forum'. As there are *far* more 'off topic' topics than *on topic* topics, of course the addition of all posts made regarding those topics is going to be greater than the number of on topic topics.

I.E. If you have 600 topics, one of which is gaming, and each one of those 600 has one post, except that gaming has 200 posts, well, gaming has the most posts - but if you put all the 599 that aren't gaming into a single topic, and call it 'off topic', there will be more posts in it than in the 'gaming' topic, simply because of attrition.