An oxygen analyser doesn’t measure the percentage of oxygen. Its sensor responds to the partial pressure of oxygen, so humidity, altitude, the weather and temperature all move the reading. Calibrate to 20.9 % in humid air, or at home and then use the analyser at a dive site in the mountains, and the same gas shows a different figure.
Enter the gas and the conditions at calibration and at measurement, and the calculator estimates what the analyser should show. Or enter what it shows to find out how much oxygen the gas really holds.