You are correct they should both be using the Common Air Quality Index (CAQI). There is a new standard the EAQI and perhaps Germany has already switched to that

You are correct they should both be using the Common Air Quality Index (CAQI). There is a new standard the EAQI and perhaps Germany has already switched to that standard. https://en.wikipedia.org/wiki/Air_quality_index https://en.wikipedia.org/wiki/Air_quality_index