At some airport locations, the location is supposed to charge an airport access/concession fee (ie a tax on travellers) if you arrive via the airport. At some locations if you are not coming from the airport (or at least STATE you are not coming from the airport and don't give your name on the bus) you can avoid the fee -- hence if you leave out flight info, you don't pay the fee. In theory, you are breaking some sort of rule (ie you are supposed to pay the fee if you arrive via the airport).
Other locations, you have to pay the airport fee no matter what.