What accounts for “dead weight?” A person who is “limp” for some reason appears heavier than when that person is standing upright. I’ve always been under the impression that the appearance of increased weight is mainly due to the awkward centers of mass of either the lifter or the person being lifted.
It’s the difference between trying to lift a queen-sized mattress, and a crate the same weight. It’s a lot harder to cope with a weight that isn’t steady.
Think about picking up your friend (in such a way that your left arm goes under their arms and you right arm goes under their knees) If their “not dead” they will keep their torso and waist semi-straight and hold them in place while someone who is “dead” will not and they will simply slip right through the middle so you have to put much more effort into holding them. Just like the mattress/crate analogy.