they are accurate because someone "zeroed" it in, or made it accurate. it is important to understand that that is only done at a specific range. for example, a rifle could be zeroed at one hundred yards, so if you want to hit something at that range, you would put the crosshair directly on it. if you want to hit something beyond that, you would need to use the stadia marks underneath the crosshair to aim above the impact point you want.