Cielo Classic Sportif

My thinking is that Deda OS bars and clamps are 31.7mm. Not the 31.8mm standard that everyone else uses. No idea what affect this has TBH.

It comes from rounding down a fraction of a mm rather than up. The size of deda handlebars is not any different than other brands (manufacturing tolerances notwithstanding, although I doubt this has any effect either).