Sometimes - usually early morning - your liver decides it will give you a little helping hand and "dumps" some glucose into your system to help you get into a state of readiness/alertness for the day ahead. If you're testing when you first wake up you'll see a rise in your blood sugar levels - which obviously isn't caused by eating. It's also often called the "Dawn Phenomenon".
According to Diane Kress's book, the liver dumps every 5 hours or so of fasting, hence her diet plan is based on eating every 4-5 hours during the day. It also happens if your body perceives your BG has dropped too low as can happen with vigorous exercise. It can be a right pain in the butt at times.