This is a common question. A leaf spring does not simply resist compression alone like a coils spring. First a leaf resists body roll in the twisting of the leaf. As an example, if you remove one leaf spring the remaining leaf would still provide some roll resistance. One coil or coilover spring provides no roll resistance. This is the reason that different bushings effect the rate of the spring. Changing from rubber to solid bushings increases the roll resistance of a leaf spring and will loosen the car in cornering without effecting the ride rate of the spring. Second when leafs are attached solid to the axles, the rear end becomes a torsion bar. This provides additional roll resistance in the rear end itself and through the twisting of each leaf in opposite directions. Mounting the rear end to the leaf with floating brackets will eliminate this part of the roll resistance and will tend to tighten the car in cornering. As a simple answer, leafs must have a softer compression rate to accommodate the increase in roll stiffness.