I suspect it is time to explain "Umpire's Call" again. After the ball strikes the pad, what you see is a projection of where the ball might have been, it isn't the actual ball because that has met an obstruction. If more than 50% of the ball is projected to hit the stumps, you can be 100% sure it will. But if less than 50% of the ball is projected to hit the stumps, the current accuracy levels cannot state with 100% certainty that the ball would have hit the stumps. Hence, you go back to the Umpire's original decision because you cannot be certain enough to overturn his call. It is a very good and fair method. As cameras get better and the projected path more certain, we could reach a day when you can be certain that even if the projected path shows a ball merely clipping the stumps, it would in reality have hit them.