There are a few possible explanations:
1. It could be an erroneous false reading. I know strips are expensive, but anytime I get something that outside of my norms I re-wash and re-test to be sure. I even wipe the first drop of blood and test the second to get my cleanest possible sample. Most times it turns out to be an erroneous false reading for some reason.
2. If it had been a while since you'd had anything to eat, or if your body sensed it was getting 'low' in blood glucose (if your metabolism is more active than normal, from exercise or whatever), your liver might have simply 'dumped' a little glucose into your blood stream. That is one of it's functions, is to feed you when it thinks you need it... For me, I can't go more than 3.5 hours or so without a meal or I get a small dump (my dumps usually put me at about a 6.2, give or take) ... Of course, we're all different...