With WUnderground, you basically have to rely on the accuracy of the weather data reported by them, and it sometimes doesn’t completely match what you observe.
The user manual explains the difference between the rain sensor, rain delay, weather adjustment, and the interplay between them:
https://opensprinkler.freshdesk.com/solution/categories/5000022938/folders/5000147083/articles/5000632569-user-manual-firmware-2-1-5-current-
page 15.