@Andy_F

In fact the 1-hour precipitations stand for the last 3 hours, as shown by the weather picker and said in the FAQ.
https://community.windy.com/topic/3361/description-of-weather-overlays?sort=oldest_to_newest
You can check it hour by hour on the Rain layer. So if you accumulate all precipitation in the 1-hour table you get an excessive total. I must admit it is confusing.
I think the accumulation of 3-hour rain is more correct.

@Suty
Please could you confirm my guess.