Observation #1: There are only three points of contact you have to control the unicycle: the two pedals, and the seat. It's very important to pay close attention to how you're using these three points of contact.

Observation #2: You can choose to put more weight on the pedals, or more weight on your seat. You can vary this weight distribution on a moment-by-moment basis, it's your choice. (and when you're mountain unicycling, you have to put your weight on the pedals in order to traverse rough terrain)

Observation #3: If you don't keep the seatpost very close (<~10°) to vertical (e.g. if you ride butt out), then it's physically impossible to put any weight on the seat. Doing so would just make the seat go further away from top-center (basic trigonometry / vector decomposition).