The problem is the friction of the tire on the ground.  It will resist movement and the tires will twist, throwing off your readings.   You have two choices

1. Roll the car forward and backward several feet everytime you make an adjustment

2. place plastic on the floor where the tires will be, and oil it.  This will make the tires move easily so slight jounceing will allow them to center for accurate readings.   Be sure to clean to oil off of the tires when you are done or you will have a heck of a mess on the ground, and paint.

 

All this is first assuming that the rims are mounted properly, not bent, and the tires are in good condition and not deformed.