This right here is all that's important.
Weather changeability isn't weather changeability as per se. It's rather the speed at which you'll go through weather patterns (the weather is scripted from certain criteria. On the test day, using the settings to be used in the race, for an event I hosted earlier this year, it went from sunny to munsoony conditions after about 45 minutes. Guess when it happened in the race?) and how fast the weather changes (rain and lack thereof) make an effect on the track.
Basically, the higher the weather changeability, the faster you'll see the rain that's scripted to come at some point, and the faster the track will be flooded, whereas a lower value will make everything involving weather slower or non-changing (0). Quite a bit similar to accelerated daylight.