Installation of the calipers

The tolerances were a bit wide in the caliper, the support ways, and in the linings backing plate stampings. The rubber band will fill a void, prevent a metal to metal tick as the parts moved up or down when changing directions.
If your calipers pistons are completely retracted and the pads drag on the rotor, you can assume the rotor thickness is to the top of thickness tolerance and the linings are also at the top of their thickness tolerance.
I never did trust rotors to be true straight out of the box. Of course that was back when a new rotor didn't come with new races already installed. After knocking in the races I would put them in a brake lathe for a light check cut. I suppose that's why I've never had the tight fit problem.