Every hour the firmware sends out a weather query. Upon successfully receiving the result of the query, it logs the water percentage number to SD card:
https://github.com/OpenSprinkler/OpenSprinkler-Firmware/blob/master/weather.cpp#L113
Whether this number changes during a day or not depends on how frequent the weather station nearby you updates the weather data. If the weather data hasn’t changed in 24 hours, the calculated watering percentage will remain the same. The Zimmerman algorithm (assuming this is what you selected under Weather Algorithms) uses the daily temperature, humidity, and rainfall data to calculate watering percentage. If you selected a different weather algorithm (such as ‘Auto Rain Delay’) then the result will be different.