Concerning Windy Maps which is a different application from Windy, it is planned to give the possibility to display imperial units:
If you look your data for Saturday the 13th of August, you see 0.4mm at 5.00AM. This is the total of rain accumulation between 2.00AM and 5.00AM. Then you see 0.9mm at 8.00AM. This is the total of rain between 5.00AM and 8.00AM.
If you go for Windy Premium, by splitting from 3h forecast to 1h forecast, you will see how the rain fall is spread by 1h in place of seeing the accumulation over 3h : for instance, you could have 0.2mm at 4.00AM and 0.2mm at 5.00AM as a potential example