One of my customer brought his High dollar Snap on dial back timing light to our shop and done some comparing it on his racecar and his light did show the timming mark jumping all the place and I got my Old school timming light and it show the timming mark is so steady you couldn't see it move.
Customer was so fustrated after he saw the diffrence and threw his timmig light far as he could and went bought plain jane timming light.