Well radio waves are to one extent or another effected by anything near them.. the surface of the ground is a major something.. I say something cause you don't even know for sure the consistency of the soil or what might be in it. Radio waves are less disturbed by air than by solid objects, like the ground. The inconsistencies are measurable, just do enough range checks with the same radio and aircraft setup on the ground and you'll discover what I have observed.
Lee