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Wikipedia does a good job explaining after the chart here.
Most people aren’t riding trains or buses hundreds of miles at a time, and there are FAR more people riding trains and buses. So per hour traveled, those will be safer.
But while you don’t take planes from say, one part of NYC to another, you CAN take trains and buses to other cities. So mileage becomes a more meaningful comparison. Sure a bus might be safer per hour, but a bus ride from NYC to LA will take many more hours than a plane ride.
I compare it to the smell of stinkbugs rather than the taste of soap.