The flight plan formally calls for a stop in Antwerp just to refuel. If that happened the plane would land at Antwerp and top up the tanks with the passengers on board before continuing to Amsterdam so from the passengers perspective they're always getting to their destination but sometimes the arrival is delayed for a fuel stop.

I personally would be annoyed if I booked a non-stop that was expected to have s top by default but I guess if it works out a high enough portion of the time it doesn't really matter