When we do camera calibration, we have to find calibration matrix $M$, which is found by first finding extrinsic matrix and then intrinsic matrix. To validate the accuracy of calibration matrix, we compare groundtruth projections $points \times intrinsic-matrix$ against prediction $points \times extrinsic-matrix \times intrinsic-matrix$. To illustrate results, I plotted the figure below. Source
Question: Why the projections from $points \times intrinsic-matrix$ are regarded as the groundtruth please?