Thrust arm torque/preload. Rear on ramps vs jack under diff?

If car is correctly preloaded, front wheels on ramps and car is level, how can front suspension tell the difference between rear on wheels on second set of ramps vs jack under the differential. By definition the center of the diff is on centerline of rear tires.

If front suspension can't tell the diff, why not just jack the diff to level the car before torqueing the thrust arms?