I'm well aware that the airport will know how many pax they will have, but how many of those pax are in cars is a harder one to forecast.
It could be that a pax who used a train last year preferred to use a car this year, or someone who got a taxi last year but wanted to drive this year, and so on.
End of the day, forecasts can only do so much. Car parking is a huge revenue earner, do you not think they would want to maximise that rather than deliberately sell themselves short, as is being implied?