Thomson posts are thinner on the sides than on the front and back, to stiffen it against in-plane bending caused by rider weight and road bumps. I don't know if this is done in carbon posts.
A high saddle causes pelvic rocking, which is in opposition to frame torque due to pedaling. With a very stiff BB and seat tube and a high saddle, I could see this net seat tube/seat post torque resulting in seatpost bending. I could see a lower saddle helping.