The published HV curve is an attempt to put some repeatable (between types) data on paper for all pilots.
It's not perfect, but with the same rules applied to everyone, many things become clear.
From my experience, the portion from the low hover to the knee is one of the more difficult to test- there is the requirement to attempt to simultaneously get takeoff power, the necessary height and then the necessary airspeed all at the same time. The height and power are pretty easy, but the airspeed system doesn't work well at the airspeeds being tested. For one type, we looked at GPS ground speed and compared it to indicated airspeed (in zero wind conditions), and found that there was at least a 10 knot error with the airspeed lagging well behind the actual speed.
But throughout that region, it was evident that the reaction of immediately lowering collective when the engine failure was initiated and also starting a flare was key. Going from nose down and accelerating to nose up and flaring was very natural.
The same airspeed for entry from above the knee is a different story - 40 knots in most helicopters from level flight at 200' means that you don't have enough energy to flare, nor enough height to get more airspeed. So that's the reason why there might be a difference between 'below the knee' and 'above the knee'.