@tessellahedron
If I'm not wrong, windy ECMWF model is updated every 6 or 12 hours. And weather forecasts can change quite quickly depending on the location, especially those with a lot of mountains around.
Keep in mind that this is "forecasts", so it doesn't mean everything will be accurate. In the old times, most weather services gave a reliability % for accuracy: For example, 80% for sunny weather tomorrow, which means there is a 20% chances that it won't be sunny at all. Usually after 3-4 days, the reliability level drops below 50% of success. And because most weather forecast companies are businesses that needs to generate revenue, they tend to not show the reliability % anymore.
For WeatherKit, from my experience, since Apple bought Dark Sky, it just became less accurate, while Dark Sky use to be one of the most accurate. Sad.