Because most border tariff points in a station have not been properly implemented, unfortunately. Genève Cornavin station is indeed a border tariff point between Switzerland and France.

You can add your journey manually; when you place Geneva in France, it won't use an inbound/outbound journey.

OR use the local train to Bellegarde/Annemasse (new zigzag via tunnel!) and book from there-it is also so that TGV from GNV are charged at INTERN. supplmt=much higher as only in FRance.

In fact from Basel this is also the case-from GNv there are certainly some KMs on swiss ground to reach border.

Thanks! We made it!