The caliper is measuring the height difference between the inner end of the hub tube and the inner race of the bearing. It is this height difference that must be filled by the phosphor bronze spacer. Doing up the castellated nut on the driveshaft, means the bearings are tightened to the required preload/endfloat, but no further, as the spacer transmits the torque to the driveshaft inner flange by the UJ.