Why there is difference in standard deviation calculated by normfit and dfittool with normal distribution?

i am trying to calculate Std dev. of histogam of image planes, when i calculate it with std or normfit command the sigmahat value is different form the SD calculated from dfittool option with normal distribution, so pl answer the question or theory behind it.

Answer 1

[Tom Lane]

I don't know anything about images. In general, to calculate the standard deviation related to a histogram of data in a variable X, you would apply std, normfit, or dfittool to the X, not to the histogram data. I hope that's what you did.

I'd expect the three to give the same answer in that case. I'd expect the mle function to give a slightly different answer, since it compute the maximum likelihood using a 1/n factor, rather than an unbiased estimate based on a 1/(n-1) factor.