As it's come up repeatedly, here's more about ( CO2 ) calibration: https://www.co2meter.com/en-au/blogs/news/7512282-co2-sensor...

.. pretty much all instrumentation requires semi regular calibration, it varies by how often and how serious you are about exact results.

Many CO2 meters are sensitive to dirt | dust | crud build up ( see article ) and should be reset semi regularly as per the manual (or by the linked site guidelines if you've gotten a cheap one with no documentation).