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.

The three hour totals are
4am - 6mm
7am - 8mm
10am - 0.4mm

The three hour totals we can cope with, but the one hour totals would require setting up floodboards as the catchment cannot cope with that amount of rainfall over that period on already saturated ground. So it would be nice to know which values are correct.

When comparing 3-hour to 1-hour rain amount figures, keep in mind that these forecast panels only show rain amounts equal to or greater than 0.1 mm.
This can therefore lead to seemingly inconsistent amounts as explained here:
https://community.windy.com/topic/12851/rain-forecasts-issue?_=1711884397761