I found the solution by capturing the the weather service request with WireShark:
Zimmerman WU - http://weather.opensprinkler.com/weather1.py?loc=49.47432,10.94365&wto="h":100,"t":100,"r":100,"bh":65,"bt":70,"br":0,"pws":"IFRTH65","key":"Wunderground newAPI Key"
My current understandig is that DarkSky uses Yesterday’s weather data, whereas Local PWS and Wunderground use the last 24 hour weather data for the Zimmerman calculation. Please correct me if I’m wrong?