US vs. Europe sunlight duration in hours per year map

As cool as this is, I'm very curious where the data is from, and am suspicious about the validity of this map. The Pacific Northwest, where I am at, has a lot of variability in sunshine in a fairly small geographical area, so the big block of samey-colors for Idaho, Oregon, Washington, and Montana doesn't seem quite right.

Compare this against the Direct Normal Irradiation map from Solargis[1] for example. DNI is approximately what most people would think of as "clear day sunshine". There's a lot more variability in most regions compared to this map, and the color bands tend to follow geographical features a bit better.

[1] https://solargis.com/maps-and-gis-data/download/world