There have been a number of ideas how to make back of pack S shaped to contour to shape of back.
But is that really critical?
If your waist belt carries a lot of load, then maybe you want the pack to contact the hips well - sort of a rectangular area against the hips - maybe extending a little above the waist belt.
I guess that doesn't apply so much if the waist belt isn't carrying much load.
And when you carry weight on shoulders, you want good contact on upper back.
Maybe you don't want contact on small of back. If you leave it open, you can get ventilation to reduce sweating.