Blood glucose is regulated up or down depending on the needs of the body. Consequently, normal isn't fixed nor is it the same for everybody. However, it's a measure, rather naive measure you'd say, of whether a person has diabetes type 2. Now that we know about normal physiological insulin resistance induced by a high fat diet, which is otherwise completely healthful, then the resulting blood glucose measurement should be viewed as normal, whatever that measure is. A GTT is a rather archaic method of determining if a person's glycolysis pathways are disrupted, i.e. we overload the pathways and see if it returns to normal in a pre-determined "normal" time. Let's see if we could do this with the pain pathways of our various organs like, I dunno, hit somebody on the head with a hammer and check how much pain he feels.

The point is that blood glucose varies for several reasons, only some of which are due to disease.