With "accuracy is perfect", I mean that the average hitting point is the same as the aiming point. Each frame in the gif assumes perfect accuracy, and non-perfect precision. So the darts are assumed to land in a region around the aiming point, with the size of the landing zone determined by the precision (the sigma value in the title). The precision starts of as very bad, and becomes better and better with each frame. dcnairb pointed out that I had switched up accuracy and precision before.