Hi there, first off, it's not a big difference, it's a difference of 1.5 which is just about deemed within the margin of accuracy as mentioned. Also, as others have said, that 15 minutes wait can make a very significant difference, I can take mine say at 6, and 15 minutes later it can easily be up at 8 plus or down at 4 plus. Stress, worry, or just running up the stairs can do it, it could be that your body thought its 3.7 was too low or going lower and helpfully gave you a shot of glucose to bring it up to 5 which is a perfectly acceptable reading anyway. I think people can easily make the mistake of thinking that the glucometers are able to give a 100% accuracy reading, but in reality it is nowhere near and it's only giving you the measurement at that precise time. Same with the Libres et al. Try using your meter to take 2 readings from the same finger at the same time and you may well get different readings of around 1 to 1.5 difference. It's not great but it's certainly good enough to be able to use it to see those foods that will raise you, if you eat a food that is too high in carbs for your body to cope with, that '1.5' difference will hardly come into it, you would see it shoot up well beyond your average/normal readings, or hardly at all if your body was coping with the carbs. A Libre or CGM, takes the readings every 15 minutes or continuously and it shows in detail the ups and downs of glucose 24/7, like many other functions of our bodies it does not remain at the same number, it fluctuates. I really don't think it's a problem, it's just that the glucometers have limitations. I (as with any insulin users) have to rely on it for dosage and it's deemed accurate enough for that so you should be fine.