WaterWatch PRO is constantly processing hourly high-resolution rainfall data from the National Weather Service. When the accumulated 24-hour amount for any hour exceeds the threshold for your trial location, you'll receive a text and email. The notification contains a hyperlink, which when clicked, reveals the data for the last 48 hours.

At any time, you can see the most recent 48 hours' worth of rainfall on the app. The View Precip History tab displays hourly amount detected for a location along with the the accumulated 24-hour rainfall ending at that hour. 

This is why, sometimes, there can be variances in the values each hour.