I'm no orthopedic expert or anything, but I've read a lot online about how most of us don't have our seat position set optimally. The angle of arms, elbows, hips, back etc all play a vital part in a proper seating position.
But then there's the physical thing. We are all different sizes and shapes. I am on the (cough, cough) rather chunky side of things and 6'1", and maybe due to my inbuilt padding or that fact that all of my MK7's have had DCC, but I've never had a issue - even on journeys of 4 hours or more. In fact I become more tired generally then my back or legs etc. However someone who is taller, shorter, bigger, skinnier than me all have their own seating requirements, and I guess (and only guess) that most manufacturers try and cater for the largest demographic when designing their seating, but unfortunately can't cater for every possible combination of size and shape.