You should be questioning the hourly data since the daily data is more accurate. It is generally better practice to take the daily precipitation and model the hourly precipitation from it.
There are a number of reasons for the precipitation discrepancies:
The method used to measure precipitation. (Weatherunderground is just reissuing data from other sources and it is difficult to find on the site how the precipitation was actually measured.)
The weather stations may accumulate the precipitation in a number of different ways. Because of these differences, the hourly totals don't necessarily add up to the same amount as reported by the daily value.
Daily rain gauges are usually more accurate than either radar precipitation estimates or hourly gauge reports.
Hourly figures are generally raw uncorrected automated radar measurements which tend to be unreliable.
Daily precipitation figures are generally more accurate when taken from an observed rain gauge rather than an automated radar station.
In general, it is difficult to get reliable estimates of hourly rainfall intensities. Here is an example of it:
"Properly exposed, calibrated, and evaluated, the customary 8-inch-diameter weighing rain gage yields hourly precipitation values with standard errors of about .01 inch, so that reliability within .02 inch can be assumed. But two identical gages 10 feet apart on a windy ridge top can differ consistently in catch by 50 per cent of the smaller. Four pairs of identical gages, exposed side by side at four sites in Santa Barbara County, California, January–April 1959, were studied."
