See the stddev_samp, stddev_pop var_samp, and var_pop reference pages in the Reference Manual: Blocks to see the formulas that Adaptive Server uses to define variances and standard deviations.
The functions are similar, but are used for different purposes:
var_samp and stddev_samp – are used when you want evaluate a sample—that is, a subset—of a population as being representative of the entire population.
var_pop and stddev_pop – are used when you have all of the data available for a population, or when n is so large that the difference between n and n-1 is insignificant.
