“Meteorologists aren’t in agreement on how to measure the Probability of Precipitation, aka that little percentage of rain on your weather app. Some use a formula for it — PoP = C x A, where C stands for confidence and A stands for area. So if there’s a 50% chance of rain in 80% of a given area, your probability of rain becomes 40%.

More commonly, meteorologists measure PoP as the chance of rain at any given point in the area they cover during a certain period of time. This is the definition closest to the official definition from the National Oceanic and Atmospheric Administration (NOAA).”

Source: https://www.tasteofhome.com/article/percent-chance-of-rain/

It’s likely Apple does something unique in how they calculate their percentages. It could legit be a fault in how they source their weather data but knowing more about the actual equation might be helpful in seeing if everyone is actually measuring the same thing, the same way.