0.2mm is a small amount. Manufacturing tolerances or grinding some off by rotating the bars in the stem clamp when it is still tightened or even heavily tightening the clamp until it ovalizes the bar will give you that difference.
I think your '70's Cinelli handlebar is intended for a 26.4mm clamp. I've got a couple of old Cinelli handlebars, but I've never put calipers to them. I use them with Cinelli stems marked 26.4mm, and they tighten up fine. I don't know why your calipers show 26.2mm at the clamp area on the handlebars, but it sounds like the two stems you mention have some problems independent of the handlebars.