In theory yes, but in practice any sensor that's not regularly calibrated against a well known reference (eg. a chamber with 0 ppm CO2) will always be off somew

In theory yes, but in practice any sensor that's not regularly calibrated against a well known reference (eg. a chamber with 0 ppm CO2) will always be off somewhat. This automatic calibration just ensures that the sensor is in the right ballpark, but if a 10-20 ppm absolute difference in CO2 concentration is important for your application, you'll need a better sensor and/or calibration procedure than the automatic background calibration. Realistically, for home applications, it'll be more like CO2 < 500 ppm, things are good, above 700, might be time to consider opening the windows, above 1000, definitely time to open the windows. For that kind of application, being off by 10-20 ppm doesn't really matter too much. On a side note, it is pretty disheartening to think that the 400 ppm CO2 background level was accurate a few years ago when the sensors were designed, and that in the span of a few years that background reference is now off.