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...one of the things you realize after you've encountered enough of them is that seat posts are not always standardized at the diameter marked on them, and frames are not always reamed to the standards you expect. So if it's an alloy post, there's no reason not to carefully sand down the portion that goes into the seat tube a skosh to get .the fit you need.

If 26.8 went in there without deforming anything, and was just a little tight, going to something like 26.4 or less is a bad idea.

I cam measure ten 27.2 seatposts out in the collection in the garage, and only 5 of them will come in at what I need on a particular frame. Which is why I have a milk cratefull of seat posts.