The figures are calculated on average load. Hence a normally full aircraft beats a train which on average will be travelling half full (no matter what it feels like on a monday morning rush hour).
So you could argue (as the article does) that if you hop onto a scheduled public transport vehicle, then your additional 'cost' is virtually zero - whether it be train, bus or air. Or even your wife's car, if she was making the journey anyway.
It's a different perspective depending on whether you as an individual are making a decision about a particular journey, or whether the decision is over which transport methods the country as a whole should support.
Of course, there is the argument that if lots of us travel by short haul air (or whatever), because the plane was flying anyway so we individually don't make any difference, then we contribute to the profitability and the likelihood that air travel will increase.
