The data that OpenSprinkler uses to calculate watering percentage is queried from the Wunderground API key, and that can be different from what you see on the web, which is queried in a different way. If you want the data to be more predictable, you may consider choosing a specific weather station near you, and choose that station as your Location. That way you know that the data is directly from that particular weather station. Otherwise Wunderground uses aggregated information from a list of nearby stations and that can be different from any particular weather station.