Referring to Blood Glucose, Insulin & Diabetes, a healthy person regulates his/her blood glucose level so that it doesn't go too high or too low. See Control of blood sugar levels.
Someone who has diabetes either has insufficient/no insulin secretion (type 1) or has insulin secretion but it's ineffective (type 2). People with type 1 diabetes have to inject various types of insulin and monitor their Blood Glucose (BG) with a BG meter. Here lies the problem. Consider the following analogy:-
Blood glucose control is like driving down a road cut into the side of a mountain. If you steer too far to the left (low blood glucose), you fall off the edge of the road and die. If you steer too far to the right (high blood glucose), you smash yourself against the side of the mountain and damage yourself. Injecting insulin/taking oral hypoglycaemic agents is like pulling on the steering wheel to the left. Eating sugary/starchy carbs is like pulling on the steering wheel to the right. Exercising is like pulling on the steering wheel to the left.
Not monitoring BG regularly is like driving down the above road with your eyes shut most of the time. You have no idea where you are on the road. Every time you monitor BG, you open your eyes.
A high-glycaemic load diet + high-medication regimen (e.g. ADA & Diabetes-UK) is like pulling on the steering wheel hard, which results in diabetics veering all over the road. As diabetics don't want to fall off the edge of the road and die, they steer too far to the right (high average blood glucose) and damage themselves due to glycation.
A low-glycaemic load diet + low-medication regimen (e.g. Bernstein, Christie, Ruhl, Cooksey, Shwarzbein etc) is like pulling on the steering wheel gently, which results in diabetics steering a reasonably straight course down the middle of the road. Regular BG monitoring allows rapid error-correction.
As low blood glucose is potentially fatal, that's why Studies show that a high average blood glucose level has lower mortality than a more normal average blood glucose level.