For whatever reason, the "Six Sigma" techniques/ideas aren't using "Two Sigma" to refer to ±2σ, instead they're referring to the width of the band around the mean.
That's apparently based on Motorola's data regarding process drift.
> Motorola has determined, through years of process and data collection, that processes vary and drift over time – what they call the Long-Term Dynamic Mean Variation. This variation typically falls between 1.4 and 1.6.
https://en.wikipedia.org/wiki/Six_Sigma#Sigma_levels