There are basically three extreme tracks a flight can take:
- minimum time track
- minimum cost track
- minimum distance track
So, if the plane has to catch up a delay, the MTT might be the most sensible option - the direct cost is higher, but it might save some people´s connecting flights. If the flight is on time, the MTT would be pointless, so you choose the MCT, which gives you longer flying times etc.
It´s not only weather.