Finger prick and CGM readings are not directly comparable. A glucometer (finger prick device) takes a one-time measurement from the blood in your fingertip while the CGM measures continuously from the interstitial fluid in your upper arm.

CGMs are intended to share your overall glucose trends over long periods and finger prick tests give you a snapshot of your glucose values in a given moment. If you were to take multiple finger prick tests over an hour, you would likely receive varying results.

As the CGM measures the glucose in your interstitial fluid, there is a slight delay of about 15-20 minutes compared to the blood measurement. This is because it takes about 15 minutes for the sugar in your blood to reach your interstitial fluid.

Like any data, your blood sugar data is not always 100% accurate. The accuracy comes from of course the sensor, but also from the software algorithms used with it. An average 10 mg/dL difference between different measurement devices and apps is fairly common since the sensor accuracy for both parties is often +- 20 mg/dL. The most important thing about glucose monitoring (if not for medical use) is the trends of your blood sugar - stability, spikes, and fluctuations.

Here is a video explaining why the two readings may differ: