That's the point - globally all air now contains above 400ppm, it's the great tragedy of climate change. You can only see less than 400ppm if you put the sensor

That's the point - globally all air now contains above 400ppm, it's the great tragedy of climate change. You can only see less than 400ppm if you put the sensor in an airtight container and add something that absorbs CO2 to it - then the value will go below 400ppm. But because that's not how most people use these sensors, they are calibrated for 400ppm upwards.