What you see in the "physical" sense of the mortar and what is actaully happening with the projectile and "barrel" are two different things. "Height above ground" should not have an impact on range. As the projectile fires in a parabolic arc across the map.
Even if one mortar was down in a valley lower than another mortar higher up. They both should be able to reach the same distance, as long as, there is no object in between them.
