You're not alone with this, Andy. Here's my take. Through years of use, that post is wallowing under stress, even minimal as may be its also a softer metal than the frame and gouges the post. You can apply anti-seize much as you like but if that seat tube slit is gouging the post, one will fight it and further mar the post. Next thing you know, it has to be cleaned up, usually by sanding, including doing the seat post itself. The same old situation is repeated through that bikes life of usage.
These thin wall steel bikes do take a beating, especially with heavier riders. Common as they age, will have some out of round. Thick wall gas pipe jobbies seem less of an issue. At least in my experience.
My solution is to correct it best as possible and trial fit the best fitting post, disregarding the size stamped in it.