Hi and welcome!
Do you eat potato, pasta, rice, bread, tropical fruit, sweet drinks and starchy veg? Because they all break down to glucose in the body and cause the blood glucose to fluctuate.
Lots of us find that our fasting blood glucose is affected by a number of variables, including (but not restricted to) amount of sleep, lack of sleep, stress levels, what we ate the night before, whether we low carb, time we get up, how active we are between getting up and testing, how hot the shower was, whether it is the weekend, or the week, meter innaccuracies, and probably a thousand more...
Really, the best thing to do is record your fasting blood reading on a daily basis, and then look for trends. Cutting carbs in the diet can help to bring the average down, so can exercise, medication and several other things.
The single thing that has the most impact on my fasting blood glucose is stress.