It is possible that the app is buggy. In another post, someone also reported that the weather data were not updated. So it’s possible that your values in the Netatmo app and in Homey are from different time periods.
But in Insights you can check if the values were updated in the last hours.
EDIT
It started to rain and here are the actual data from my weather station in comparison to the values in Homey. As you can see they are equal:
Probably “Rain now” is comparable with the symbolic raindrops. In another iOS app (myatmo), a value for “Rain now current” is given.
So the original Netamo app presents less data (at least numerically) than would be possible.
How the values for “Rain now” are calculated exactly, I’m not sure. Especially because the values are updated only every 5-10 minutes.
This is a screenshot from the actual rain values in Insights:
