Why is there a time range for holdover times charts? 2016-2017 FAA Holdover Time Guide

The FAA publishes holdover times for each winter season and gives a time frame for how long each type of fluid should protect the aircraft from further contamination.  

Why is there a time range for these numbers?  Are there different procedures to follow if your takeoff will be before, in between or after the specified time?
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.