I have been told that the finger is always the most accurate location because it reflects changes quicker. I'm not sure why, but I believe it is because of the type of blood vessels there. We use the forearm most of the time for my 8-year-old son. His endo told me that the forearm numbers will lag behind what the finger would show. Therefore, it you are at a time when the BG is quickly changing (like after eating or after giving a bolus or injection of insulin), the finger is your best bet. If it has been a while since you've done anything to change your BG, the forearm should be fairly accurate. So it's possible your BG is dropping at the time you're checking and that is why the finger shows a lower number.