Also if you want to start talking, yes...you will have more compression on your spine in a seated position, but it's a stable compression because your body is in a static position.
standing you are not stable in any way, and you have far more risk of pulling a muscle as your body is forced to stabilize that load as you press it up. every muscle in your body is firing like crazy to control that load, one wrong move and you could be injured, also a good way to get a hernia
and i wasn't taking about spine pressure to begin with, but about stabilizing your trunk so as not to pull a muscle, and a safe ROM