The results of the curve can be affected by several factors, and measuring the curve at the veterinary practice can give an inaccurate portrayal of what is occurring at home. Not eating or stress (causing hyperglycaemia) may occur at the vet clinic. If your dog won't eat at the clinic then feed your dog at home before going to the clinic for the blood glucose curve.
In addition, it is not uncommon for curves to vary from day to day because many things can affect blood glucose levels such as appetite, digestion, metabolism, exercise, hormones, stress, etc., even in well-regulated dogs.