I would imagine the founder and administrator should have the highest post count when he or she is first starting their forum as you need to fill it out to begin with and it could be just them posting at the start but overtime as they develop a larger community with more active members then I don't see why it really matters then, as long as they are still actively posting, adding new discussions and moderating the forums. 🙂
I think on most forums the admins have more posts because most admins post a lot to build up the contest before releasing there forum. Actually, I have yet to see a forum where the owner doesn't have the most posts 😛