Purely speculating, but it may be the expected duration of rain. So 30% in one hour means on the average it will rain 18 minutes within that hour and 50% of 15

Purely speculating, but it may be the expected duration of rain. So 30% in one hour means on the average it will rain 18 minutes within that hour and 50% of 15 mins would be 7.5 rainy minutes. My real guess is that they have independent models for different time resolutions.