simmple, a very good question; interesting.
I do not have an answer, but 35 ft was allowed for steep approaches.
In part, this, like conventional approaches with 50 ft TCH, was the reference for landing performance.
I recall that there are airfield constraints such as obstacle clearance surface, 1:40 ? (I cannot recall).
Also that this slope was cut off before the runway, but not sure of the ref point for the slope.