Well its like this. The pillion is thin, a little narrow and kinda firm which contrasts with her backside.
The Sundowner is wider, and softer which is more of a match with her backside. Don't tell her I said this.
Also, with the detachable backrest on, the sideplates dig into her rear. Since the Sundowner is thicker, it raises her up a little and the side plates don't bother.