Don’t forget that the finger sticks also have an accuracy tolerance of 31 points. So if actual blood sugars are 100, the finger stick could be reading 69 and the Dexcom could be reading 120 and they are both right!
Then add in the 10-minute delay it takes for the blood sugars to absorb into the interstitial fluid under the skin, which is what the Dexcom is reading, it could be even further out if your blood sugars are falling!
The best way to test accuracy is to check when you blood sugars are stable. Before meals etc. when your Dexcom is indicating a nice straight horizontal line for a good 30-minutes or longer. It also wants to be indicating ideal blood sugar ranges between 80 and 180. Then do at least 2-finger stick blood tests to see if the Dexcom is close. Ideally it should be within 20% of the finger sticks. Take an average of the finger sticks if you have to or discount one of the results if you have to. If you have bags of free test sticks like I always seem to have, just do 3 or 4-finger stick tests and then you will have a much better idea which results to rely on and which to ignore.
It’s also worth remembering that your finger stick blood test meter also needs calibrating from time to time using the test fluid the manufacturer normally provides with the meter! No point questioning the Dexcom, if the meter is so far out, it is not worth considering.