Why is it better to have a rifle shoot a little low rather than a little high?
Sights have to be the appropriate height for a rifle to shoot where you are aiming, and there can be some variation between barrels. It is better to have a gun shoot a little low, so you can file down the front sight to bring the point of impact up, than to have it shoot high, which would require building up a taller front sight or filing down the rear sight.