This site likely operates the same as Climate FieldView. They essentially take data points and then use an overlay to smooth a general area depending on those points. They can be generally close but also wildly inaccurate at times. It really just depends where the data source is in relationship to where you're seeking data for and how sharp the gradient is. In a year like this year 1 mile could be the difference between 2 inches of rain. However they are generally a good way to tell if a general area got a lot of rain, or a little rain.
For example, I dropped a pin on my house and it says 3.27" and my gauge this morning had right at 4". I'm assuming there is a data point around the research farm West of town and at the Airport, so seeing how the totals drop off to the East side of town it makes sense that my average would appear lower due to the smoothing overlay.