It would appear that we've allowed this to get more complicated than it really is. For practical aviation purposes it's nice to know that if you are in a HOGE at say 800' and you lose your (single)engine, you can fully lower your collective from a high position in order to make a safe autorotative landing.
If you are in a descent (lower collective position) then the lever has less distance to go to achieve autorotation. this would be represented by a smaller HV diagram. But who would want to look at an infinite number of HV diagrams during flight (one for each possible lever position or phase of flight)! You have to keep it practical and useable.