As mentioned, the refacer operator's experience has much to do with using the machine. He can alter that arc and its center depending on cam, pushrod's length,stem height, rocker ratio,etc.
It’s probably safe to say that individuals with this experience will soon be extinct. Assuming the machine has adjustability how does the operator determine the required arc and position? Does he set it up to get close to the existing wear pattern? If the rocker pad is not parallel to the head face (front to back) then I could see how the off center contact would attempt to rotate the valve in the same way that this offset contact between cam lobe and lifter induces rotation. Perhaps you folks are not on the same “plane”.
