I'm an occupational therapist. I help people design ways to change whatever it is they want to change in themselves, through daily practice.
Lest anyone feel the urge to say "why change people?", it is in our nature to strive to improve ourselves - and every living thing changes every day anyway.
I agree that there is no "normal", yet there it is on the graph. I am often called upon to explain that graph to people, so here is my take:
If someone is coming to me, they want help. Lots of people want help, for lots of reasons (it goes back to that social, tribal thing). Most of the time, they'll want their health insurance to pay for that help, only their health insurance doesn't know them, me, or their results as anything more than a number. They won't offer the help because you want it, and feel you can improve your life. To get help, we are expected to show you "need" it (you are the "some people" in the area you'd like to see change), and show that you "improve" with help (get closer to the "most people" range.
I'm annoyed with the system, because individuals aren't and don't strive to be in that middle zone - but I know why it exists and the intention is to help people.
For things like iron levels, blood glucose, and more, striving for the middle of that graph makes total sense. For things like reading speed, muscle tone, and balance, it's just information, insight that can be useful.
Diagnoses aren't created to make people conform, but they are created to draw boxes around who can and can not get help - which may actually be worse.