Here's my logic: think of 2 setups, exaggerated to illustrate what's happening.
First setup, the shock/spring is vertical. An inch of vertical rear axle movement requires the shock/spring to compress 1 inch.
Second setup, the shock/spring is angled 45 degrees away from vertical due to the lowering bracket relocating the bottom of the shock/spring rearwards. An inch of vertical rear axle movement requires less than an inch of shock/spring compression. So this would happen more easily, resulting in a softer feel.