There’s a concept in Reliability Engineering known as the bathtub curve.
The idea is that there’s a middle region where failures are fairly constant. Before that time there’s infant mortality -where we buy something and it’s broken in the first few days. Finally there’s a range where things are worn out and fail.
Causes of Mortality
There are similar curves of mortality rates for various causes of death. HbA1c numbers follow the same concept. Too high of an HbA1C number is bad (diabetes). Too low of a value is also bad because death from hypoglycemia is possible and there are other diseases which might produce a very low blood sugar. In the middle there’s a sweet spot with some “healthy” range.
For example: All-cause mortality HR for HbA1c values:
Too high of an HbA1c is bad. Too low is bad, too.
See also Hazard Ratio.