How is the standard deviation calculated in the control chart?

[Ashley Lewis]

Can someone provide the formula for the standard deviation (light blue shaded area) on a control chart? It is strange to me that it does not stretch equally above and below the mean like a true standard deviation calculation. 

[Erik De Bos]

I think you are assuming a normal distribution, but with estimation, that is not the case.

The chance that a user story takes shorter than the estimate is lower than that it takes longer, so, therefore, the variability differs above and below the mean.