I think the Explanation is bit confusing.
if you give any numeric value to the function without any from clause it will give NULL.
"The STDEV function uses all of the values to calculate the standard deviation. There must be at least two values in the field or the standard deviation will not be calculated and a NULL is returned."
example in my PackageTable the standard deveation of pa_rates column can find using
select stdev(pa_Rates) From Package
for the above Query i will get some result
but if i cange the Query with some where condition,where i will get only one Row (sample) then the stdevp will results the null value
select stdev(pa_Rates) From Package where pa_PackageID = 1