The normal range is based on a normal population, like male adults, female adults, children and their varying ages, etc.
As others have said, the reason for fasting is to prevent interference with certain blood tests – for example, alcohol affects triglycerides a LOT, like several hundred units. For some tests, I remember learning not to chew even sugarless gum because it causes your stomach to produce gastric fluids. As far as how strict the fasting should be, some say water and coffee are ok, others say nothing, some say 10 hours, some 12 hours; I’ve never heard of 4 hours, but haven’t been in the field for a while. When I fast, I do 12 hours, water if I really need it.
lab tech for 12 years