You forget that HR affects CS as well. Aim is also a very large component on maps that are already difficult (already a high aim score), since speed score doesn't factor into HR at all.
In that case, performing as you would at AR9 doesn't apply. If I modify a map and only change it's AR to 10, I can perform just as well as I would at AR9, but that's not the same as what HR does, which increases the accuracy requirement (OD) and the aim requirement (CS). AR alone is negligible by comparison (and pp agrees too).
You try modify a difficult map that you have a good score on, and change only it's circle size (multiplied by 1.3x), and it's OD to 10, and tell me if you can perform exactly the same as the original map with the same level of consistency. These are real factors of difficulty and should also be considered.
You see, I've actually selected practice maps since about a year ago, where I've only changed the CS, and even setting it up by just 1 makes a drastic difference to my aim consistency. The reason I did that is because my aim isn't good, and it definitely needs work before I can play HR properly (acc isn't great either tbh).