Even if you gave everyone the same model tester from the same batch run , you'd still get different readings .
There are no two motors that are absolutely identical , let along wishing for identical results from testers from one end of the run to the other .
And , that's not considering all the different hands involved in any testing or any differences in atmospheric conditions .
Sure , results may be pretty close , but so are they from different brand testers .
Simply way too many variables .