In fact, it is not unusual that Radon levels vary greatly even throughout the day. Radon varies from building to building in the same area, and it can vary from room to room in a home.
This depends on temperature, humidity levels, atmospheric pressure, the structure of the building and the time of the year (usually, but not always, radon levels tend to be higher during winter time compared to summer) and a number of other factors that influence Radon's behaviour.
This is the reason why we at Airthings recommend you let your device measure your indoor Radon levels for as long as possible. Over the long term, the value that really matters is the long-term level on your display, as it is the average level of every sampling taken by your monitor since the start of your current measurement.