Last time I posted something about this, it was downvoted to oblivion. Glad to see that didn’t happen here. There are some statistical quirks that make this obvious once you think about it though.

Planes tend to make long trips, so anything measured on a per-km basis is going to be wildly diluted. Something like a car will tend to make much shorter (but more frequent) trips, so the per-km stat won’t be diluted. But inversely, the per-trip stat will be diluted with cars, purely because they’re used much more often.