What was his mistake?
He had a different number of points to the left of the vertical line than to the right of the vertical line.
He had a different number of points above the line of best fit than below the line of best fit.
He didn’t approximate the center of the cluster located on the left side of the vertical line and of the cluster located on the right side of the vertical line.
He didn’t connect the centers of the clusters on the left side and right side of the vertical line to produce the line of best fit.