Considering that you are the one who claimed that seatposts vary all over the place, I find it rather interesting that you claim my experience is “opinion”. I don’t measure posts on a regular basis but I don’t find that there is enough variance that once a post size is known that any other post of the same size won’t fit. I certainly have never found only half of all posts of a given size will fit in the same frame.

I have 11 bikes and 2 frames in my garage. One of the bikes and one frame use 30.8mm posts. The rest use 27.2mm posts. That’s 9 bikes with that post size. I have swapped posts around on all of them. Not a single post has every not fit in any of the frames. Not too tight or too loose. Each has fit exactly as I would expect it to fit...i.e. just right. I have exactly the same experience with the 30.8mm posts. I have the same experience at my co-op where I replace a lot of posts. That’s not opinion. It’s fact.

I replace so many posts at my local co-op that I’ve gotten pretty good at guessing the size and being close in most cases. And, again, if the bike takes a certain size, all of the post marked with that size fit like they were made to fit.

And, since I know you’ll never believe me on anything, check Andrew R Stewart’s link. He saw some variance but , honestly, it would be so small as to not make any difference in the way the posts fit in a frame. He certainly didn’t seem to find that 50% of the posts were out of the standard.