@Melgar unless your days are absolutely identical, your blood sugars will vary.
When I say identical, I mean, get up at the same time after the same amount of sleep, eat exactly the same, drink the same, exercise the same amount, have the same amount of stress, the weather and amount of sunlight to be the same, if you are female, I hope you don't have monthly variable hormones, you need to take exactly the same medication and be equally healthy, ...
I could go on but the things is that there are many things which can affect our blood sugars so it is not surprising that they vary from day to day.