Why is there a time range for holdover times charts?
The document answers your question - Look under the table, in the section titled "Cautions:" --


   The time of protection will be shortened in heavy weather conditions. Heavy precipitation rates or high moisture content, high wind velocity, or jet blast may reduce holdover time below the lowest time stated in the range. Holdover time may be reduced when aircraft skin temperature is lower than outside air temperature.


Thus each category has a range of holdover times - longer for the most moderate conditions (barely enough snow to show up in the METAR), and shorter as conditions become more adverse.
For example if we split the difference in the "Rain on Cold Soaked Wing" category, Light Rain (-RA) would have a  holdover time around 5 minutes, Regular Rain (RA) would probably be around 3 minutes, and Heavy Rain (+RA) would wash the fluid off in 2 minutes or less.