Because of the geometry of the suspension that is incorrect. The A arm attaches to the frame on one end, and spindle on the other. The coil spring assembly is in between. If it was exactly centered then the lift would be twice the spacer.
I checked this when I got home yesterday. I measured the distance from the top of the coil spring to the bottom of the shock mount. Then I jacked the front end of the truck up 2". I measured the distance again and it was 1 1/8" longer. From this I can conclude that the coil spring is slightly closer to the spindle (where the tire connects) than the frame.
For every inch of lift the spacer needs to be 9/16"