When I calibrate the Dexcom G6, it does not collect the exact value

Good night.

My son has been carrying the Dexcom G6 for about 15 days.When for some reason we have calibrated it, the app corrects the deviation, but not at all.For example: if the app gives a value of 40, and the capillary glucose is 100, when introducing the calibration, a value of 80 appears in the app, not 100 which is what it really has.If we want the value of 100 to appear, the value must be introduced twice.Has someone happened to?Do you know if you have any justification?

Thank you.