There is No comparison to how someone chooses to sit on a BENCH SEAT vs the restrictions of a BUCKET SEAT/Jump Seat. It's like sitting anywhere inside a Caddy vs a Mustang front bucket seat. JFK had plenty of Lateral space options to use and therefore shift his sitting position. The back brace JFK was wearing also may have resulted in his altering his seated position. Wearing a back brace while in a seated position for an extended time period is very uncomfortable. And NOBODY knows the exact seated position of JFK at the moment the bullet entered his BACK.