when was the last time @ 1000ft on final or 500 ft you flew over a hilltop etc on landing
Try Pago Pago ILS in American Samoa. Approach over water over rising ground on the coast the radio altimeter drops rapidly to 300 ft then back to 800 ft. Try the 1000ft call RA approaching over water to the the island of Nuie in the South Pacific that stands around 400 ft amsl. Get around a bit and there is no shortage of undulating terrain under close in flight paths on final. Port Vila runway 28 where the 1000ft AFE call is 600 RA.
Read the 737 FCTM where the recommended call by the PNF is "500 Feet" and the condition is 500 ft above field elevation - no reference to radio altimeter reading.
It is clear that the call of 1000 ft and 500 ft in these respects are above airport elevation in terms of barometric altimeter reading - not radio altimeter.