There is a certain procedure that is supposed to be followed when doing an alignment. Set up the car with the wheel sensors, do the rolling comensation, put the brake pedal depressor, THEN, do the caster sweep. its an option on the screen when the alignment machine tell you to center the steering wheel, takes he measurement, turn the wheel all the way to the left, take the measurement, turn the wheel all the way to the right, take the measurement, and then center the steering wheel, lock it in place, THEN do the alignment. With both the brakes held and the steering wheel centered and locked in place... This is the procedure for about 90% of cars out there ( some cars have steering wheel pressure sensors that cannot be locked down )