I wouldn't be overly concerned. Another big sign of diabetes is frequent urination, which she doesn't seem to be doing. Her numbers were in the range- either way, think you'd be fine- if she ate a lot of sugary food before and she still had an 80, things are working. If she would have come up high, they would have most likely requested a fasting level to be more accurate. When my newphew was diagnosed at 11, he had the classic symptoms of frequent urination and endless thirst. When they tested his glucose, his numbers were sky high.
I know that, this time of year, I tend to crave much more water/liquids than I do in the winter- which is common with the heat. I would also think it might just be a growth spurt.
I'd keep an eye on it- and trust your instincts, If it continues or the symptoms increase, have them recheck it. (I work in the medical field and it can happen on occasion that they test negative, only to test positive some time later- though this isn't common.) All in all, I wouldn't be too worried. Good luck