Or different aircraft.
It just seems odd to me that it can be different for passengers in other parts of the plane. I need to read up on it, as I feel the need to understand!
Edit:
OK. That didn’t take long. It’s just based on the floor space in the plane taken up by each passenger, which is an arbitrary measure rather than any scientific calculation.
It might work for making first class passengers pay more in offsetting (which is probably a good thing), but it fails my logic test!
You can of course fly EasyJet and be totally carbon neutral
https://edition.cnn.com/2019/11/19/business/easyjet-flights-carbon-neutral/index.html