Why it is done this way
The reference range is not a target
It is the middle 95% of whoever happened to be tested at that laboratory, sick and well alike.
Sitting at the very bottom of normal for vitamin D, ferritin, B12 or free testosterone is a common reason someone feels terrible while being told nothing is wrong.
We test widely, and then we sit down and go through what came back. The interpretation is the part with value.