Do you take into account the flexibility of different materials, if one is stiffer than another one is going to work better than the other with less pressure.
I always assumed this was a major factor, more expensive calipers are probably have a better, stiffer material composition which translated inrto better braking...or less loss of force through flexing.
Pads will have a huge inpact too of course, but looking at above posts, Tektro calipers for instance, while they're ok they can't compete with better brands. It can't just be about the pads, cheap calipers, cheap materials.
And then, cables ?i'm sure some are better, some are worse, all impacting on braking force.