Why the rigid cut off points in HbA1c for diagnosis of type 2.

I umderstand an HbA1c result of 6.5 mmol/mol is a diagnosis of T2 and once you reach that point there is no going back: it affects insurance and all sorts of other things and you are permanently on the register at the GP. I had a result of 5.9 a year ago and made many lifestyle changes including losing weight - [high fat and low carb] - and exercise and am now slim with a BMI of <19. I went from weighing 72 kilos to 55 kilos in 12 months. Certainly not your average type-2-diabetic-waiting -to-happen. I actually didn't know the HbA1c result at that point and did these changes for hypertension and arthritis. I also have borderline high cholesterol.. Now I've had the HbA1c done again and it is exactly the same. My dr says this is prediabetes and only the lifestyle changes I made have prevented it rising over the past year. He now says it will rise as there is nothing more I can do diet wise, and when it reaches the 50 mark he will have the metformin waiting for me!! Maybe it's his idea of humour ?? He also says thin people develop T2 as well. Perhaps I should add that I am 71 yrs.