I suspect you measured the top ID of the seat tube but not the "ID" an inch or two down. There's a lot of potential distortion from heat and such that can change the ID shape and/or internal circumference (note I didn't say diameter). This is why when folks measure the seat post size they often don't end up with a post that fits the entire insertion amount that the post uses.


The best way to size a post is with trial and error, plain and simple. No possibility of error or assumption. The post either fits in and is loose or able to be tightened in place or it won't fit in enough. better shops keep a collection of posts in different sizes for this testing.


Also know that the post's marked size is usually a small amount under the marked size and can also vary from post to post a tad. Once again the proof is in the actual fit. Andy