Words aside, the demonstration is just adding together relatively small errors that add up to something that looks significant. The sides of the dress are loose around her torso in the reference photo but form-fitting in the sketch, the overlaid sketch is scaled too small and doesn’t match her pose, extending her lowered neck and leveling her angled shoulders. Adjusting the scale, and accounting for the draping in the outfit and the pose of her head and shoulders, there really isn’t any difference other than the arms being drawn too long.