The sigma level in industry traditionally refers to the 'bandwidth', not the distance from the mean, so what you are thinking as 1 sigma is actually 2 sigma. h

The sigma level in industry traditionally refers to the 'bandwidth', not the distance from the mean, so what you are thinking as 1 sigma is actually 2 sigma. https://en.wikipedia.org/wiki/Six_Sigma https://en.wikipedia.org/wiki/Six_Sigma