"First of all, machines like this are electronic devices that will display whatever number they are calibrated to show. It has very little to do with accurately measuring actual power of a punch.
Secondly, the reason most people with very powerful punches typically score low is because the target (bag) is mounted to an arm that is attached to the machine vertically above the target. This means that the straight, horizontal punch drives through the target, then the target rotates upward in an arching circle. The level of force is registered by how hard the ball slams upward into the machine. This counter-productive to the correct way to punch a vertical target, or a person who is grounded by weight at their feet below. Nevertheless, and device that measures impact of a linear force by rotating in a circle upward is not going to be accurate. The arm should be directly behind a flat target, and measure like a scale you step on, but horizontally backward away from the punch.
The trick of devices like this is adjust the angle of your punch to cause the arm to fling faster and harder upward. A proper punch from the best boxer or Martial Artist will never register high. I break three solid boards with a punch, and have used the same type of power on one of these machines which registered lower than teenagers with no training playing around with it. Only after a few tweaks of the angle, was I able to achieve a higher score. I would never punch in real life like what this machine requires to score high.
https://www.quora.com/Who-can-get-a-perfect-score-on-those-boxing-arcade-machines