It is a Common Phenomenon such as the Wheel's Deformations or the Edge of the Rim out of Flatness . Therefore, Compensation is very Important when Alignment.
Compensation:Means take the Value of Wheel's Deformation and Clamp's fix to Rim's Condition into the Final Calculation So We can get more Accurate Data.
This is the Reason Why SemiMechanical Alignment Data Inaccurate Because they do Compensation based on Mechanical principle,and Personal Self about Reliability and Accuracy ,The Final data Difference will Inaccurate.