It’s also worth getting the verniers on your post... I must have about 30+ ‘27.2’ seatposts to hand and running the gamut from £10 cheapies to £100+ Ti jobs, and I can count on one hand how many of them actually measure 27.2, most are very slightly undersized, and likewise a lot of frames are normally reamed just over whatever their nominal ‘size’ should be.

If you have a slightly undersized post, coupled with a slightly over-zealous reaming/cleaning at some point in the frames history it’s entirely possible you could be 0.1-0.2 out of tolerance.

Ive got one the other way, a Worksop built Raleigh that apparently only got reamed to 27.15 at the factory and is consequently a pain to fit posts into. I keep meaning to ream it out but my reamer needs new cutter as totally worn out now after decades of abuse.