"Upper body" is where you went wrong. Anatomically, we don't talk about an "upper body". There is the shoulder girdle (collar bones, scapula, manubrium), thorax (chest with ribs), the abdomen (soft, supported by the lumbar spine and flank muscles (aka fajitas, or skirt steak)), and then the pelvis. Your lumbar spine is in an entirely opposite position (flexion vs extension) when sitting vs standing because it has to accommodate the connection between your torso and your pelvis. Your torso's position may be the same, but your pelvis is clearly rotated anteriorly while sitting compared to standing. This puts different loads on the lumbar disks and the strap muscles of the spine, some of the biggest muscles in your body (in pigs these giant muscles are bread-loafed to make pork chops - that bone in the pork chop is half a lumbar vertebrate)