Why was the normal blood sugar levels 80-120 before and they made them lower?

Because type 2 diabetes mellitus becomes a significant health issue so we want to detect this disease earlier & hope we can halt or reverse the abnormality. With that objective, we put glycemia of 100 -124 as borderline results, which is not high enough to diagnose DM but changing the lifestyle can halt or reverse the progression before it's too late. The same concept applies to HbA1C 5.7-6.5.