To isolate the problem, the first thing to check is if the weather algorithm is outputting reasonable numbers. To do so, open a browser and type in the following command:
http://weather.opensprinkler.com/weather1.py?loc=xxxxx&key=xxxxx
substitute the first xxxxx with your location (such as zip code), and second xxxxx with your Wunderground API key. The output should contain a number called scale=xxx and that’s the percentage it calculated.
The weather algorithm is explained here:
https://opensprinkler.freshdesk.com/solution/articles/5000017312-using-weather-adjustments
In your particular case, it could very well be that the weather condition happened to result in 100% (humidity is slightly above 30%, and temperature is slightly above 70F).