Because the concentration of glucose in the blood is lower than in the serum/plasma, differently calibrated devices cannot be compared.
Each approved meter has a deviation of +-15% to the value of the laboratory. This is according to ISO 15197:2015 standard. If you are comparing 2 different blood glucose meters there may be differences in the extreme case to 30%. That does not mean that a wrong measures of these devices.
If you want to ensure is whether your device properly measure, measure your blood sugar fasting on the device and at the same time about the lab to the doctor. Compare the results. Here the result of the device must not exceed +-15% differ from the laboratory.
Please see this Blog: https://onedrop.today/blogs/blog/accuracy-blood-glucose-meter