All sorts of things could be happening there.
It is a personal opinion but I don't think that fasting readings are very useful. The reason is that if you fast the liver will provide sugar and you have no idea when that happened in relation to the test. The liver dump is often called the Dawn Phenomenon because it happens most often when you wake up so that may account for your fasting reading.
What you had for breakfast is important since if it was carb free I wouldn't expect much of a rise and since you have now eaten you will not get another liver dump. Your reading does go down a lot but it could if you are active during this time.
What you had the night before can enter into the equation too.
