You are less stable with a feet up bench or larsen press, so it’s harder.

Right, but is this because your brain limits effort when it senses you are not fully 100% stable? (i.e. central governor theory?) I noticed this yesterday when breaking branches off a low-hanging tree. When I bend lower (like the start position of a squat), I am able to apply drastically more force in my arms, wrists, and forearms.