@PrinzEisenherz1, please be aware that the OpenSprinkler Weather Service in its newest version 2.0.1 provides a different Weather Underground interface.
The weather observation is calculated over the last 24 hours and not for the last day. The values given by the system diagnosis are therefore different, if compared to the Weather Underground website. Please have a look at the corresponding Weather Server GitHub repository for details.
I don’t know why this has changed? In my opinion, using yesterdays weather data might be the more preferable solution for the Zimmerman Watering Method.