Well, shit. There goes my grand plan to write my conclusion for my dissertation before the X Factor this evening (got to love high quality broadcasting) I have 20 minutes to write about 550 decent words
I can answer that one! Each news company uses it's own source to measure precipitation. The National Weather Service is one of the big organizations that does that, but there are hundreds of others. And these firms (and sometimes normal people who volunteer for the job) are located all over, where the amount of precipitation is slightly different. Also, these companies measure over different time intervals (from midnight to midnight, 6 p.m. to 6 p.m., etc).
Oh, and as far as future forecasts; well, these meteorologists use different predicting software.