When operating machinery, you have to keep within certain limits. Even when visual, you do your circuit at a certain height. Why? It keeps you clear of houses and obstacles yet at the same time close the airport. If you flew your circuit 1,000' too low, what would happen? Now accepting that 100% accuracy cannot be achieved, what variation in your circuit height/altitude would be considered acceptable? We will call this variation "scale deflection"; it's not correct but it makes the point. When flying on the clocks, each and every one has to stay within certain limits. Plus or minus x knots, feet, RPM, fpm etc. Fly outside the limits and you may bump into the ground (because you can't see where it is).
PM