My point is that his map doesn't do a good job of showing that, when rainfall differences are pretty dramatic. Something like this shows it much more clearly:
Cam here to say the same thing, the map appears to be overly smoothed and does not capture the sharp rainfall changes that often occur due to the local topography - e.g. when going from the windward side of a mountain to the leeward side.
For example, according to the map the rainfall at world's wettest place Mawsynram, is about 4300mm, which is about 35% of the the actual.
https://en.wikipedia.org/wiki/Mawsynram
https://www.eldoradoweather.com/climate/us-states/oregon-ave...
As well as regional differences like the Wallowas vs SE Oregon.