The weight is not lower!!!!! The point of the centre of gravity (or centre of mass if we are talking about accelerations caused by walking) being independent of the points of contact is true for all circumstances. Of course balancing somthing on your head is difficult. That's because 1). the centre of mass of the object is above the centre of effort which is a inherently unstable 2) the combined centre of mass of person and object is higher becuase the load is higher. 3) the head is not generally flat on top so it's hard to keep the object from wobbling.
Back to the case of a rucksack.The belt/hips take most of the downward force for a framed rucksack. But the sideways forces caused by the mass being accelerated and deccelerated by all the wobbles, stops and starts of walking has to be corrected by the point of contact higher up as well as the belt. The balance between these two has to act through the centre of mass of the loaded rucksack. The position of the COM is determined solely by the way the rucksack is packed.
These total corrective forces which need to be applied to the load in the rucksac don't change. The moment of the forces about the foot on the ground required to control the rucksack don't change however the pack is strapped to the body. The total upward force has to balance the weight of the rucksack, the total moment applied to the rucksac (taken about the foot or any other point) has to balance the moment of the acceleration of the centre of mass of the rucksack about that same point. You can move the points of contact about and the lateral forces change as a result, but the moment about the foot stays constant. Sorry, but its just basic physics / mechanics.
I believe the framed is generally more stable, but I think it's because the points of contact on the sack are more rigidly held in position so the rucksac is prevented from swinging. That means the centre of gravity of the rucksack stays put relative to your body and so doesn't alter your balance point.
All this is pretty moot really. we all know to pack the rucksack weight low down for stability and higher up for aerobic efficiency. Stop the rucksack swinging too.
Anyway MrsSanta says she loves me very much but it's time to stop being a physics nerd and go out for a walk. She says using kg to measure weight is not a criminal offence and that Sheldon from the big bang theory is not a good role model for a grown up.