Why Is There No Salary Standard Deviation on PayScale?

I am sometimes asked, "why doesn't the PayScale Salary Report and Research Center show the standard deviation of the data? (See Wikipedia for the (useless) mathematical definition of standard deviation.)

People are are interested in the standard deviation, because it attempts to give the typical variation in salaries. The first thing they calculate with it is a typical range of salaries, by simply adding and subtracting it from the mean.