@josh.greenfield 's question was answered here:
@LaFleur . Individual hours of TMY EPW files are not meant to be indicators of typical climate conditions on that hour. They’re only really meant to be indicators of climate over the whole year. My guess is that some of the German EWPs are using weather conditions from a certain historical year and the others are using a different historical year. On one year, the conditions were cloudy for that hour. On the other year, the conditions were sunny for that hour.
If you want to do an apples-to-apples comparison of clear sky conditions, you can use the Wea from Tau Clear Sky to make a Wea that uses a standard clear sky for each of those locations.