I’m assuming that that field internally is just a long string and they parse it and then count the occurrences. So if something, let’s say depending on a different API, is getting reported for both of the features, it will be counted double.

Ah apparently, it is more common to occur naturally in the countyside because the NOx from fossil fuels will react with the ozon to create NO² and O².

on that topic - where does the O³ come from?