It is my limited understanding that no EM signal drops off as vertical wall at the exact edge of spectrum, and radar altimeters behave within actual defined requirements and regulations. The land analogy is useful exactly to the point where it is deceptive / misleading (no ill intent is assumed here! We are all just trying to figure it out :). Land behaves like land (nice and discrete and binary with a thick clear line) and waves, well, like waves :-)