I use the 10-14 day guide as well. I agree, its not written in stone, merely a "average". Still when planning a camp day or a "hard" day, it doesn't hurt to have the "average" on your side.
 I would say that in the 5+ years I've brought the 14 day scale, it's been pretty accurate maybe 75% of the time.
 The biggest discrepancy I notice is in the amount of rainfall?
Light showers might be Thunderstorms or vice versa. Occasional showers might be all day. Temps seem too be pretty spot on.