I don’t know why it’s working like that.
But in my opinion we should’ve the ability to set weather not only for “variable dry” and “variable wet” but a mix of both.
For example in an endurance race we could start at clear, sunny weather, in the middle it will turn to thunderstorm with rain and until the end it will change back to something dry again.
It would be great to have every weather available in a race because of the need to change tyres.
This is something Forza Motorsport ended and had it in the past.