https://www.modernbike.com/seatposts+diameteris25.6-

and there is a really high probability that the actual post size is 25.4mm... measure the seat tube again, with a better caliper or micrometer. ignore the bad caliper, and look for an uneven slot in the frame tube...
i also measure in the=ree directions on the OUTSIDE of the frame tube, at two different heights... near top, and about 4 inches down from the top. this can expose an Over-Clamp situation.

there are 8 or 9 bikes listed on Sheldon Brown's Database of seat post sizes that used a 25.8mm seat post... none used a 25.6mm post.
three of the 25.8 posts fit old/mid school schwinns, AKA: pre-takeover.schwinns.
a couple fujis, a miyata, an "Ancient" Iron Horse, and a diamond back were also list... and one trek MIGHT have used a 25.8 post....
look up your bike.. if it's an older one.
M-Z .. there is also A-L...
https://sheldonbrown.com/seatpost-sizes-m-z.html#m
someone may have sanded the frame tube to death trying to get out rust, or the top part is flared from whatever, and a larger post got stuffed in...
25.4mm is the second most common seat post size.... behind the super-common road bike post of 27.2mm
the next occasionally used size, above 25.4mm, typically found, is a 26.0mm.. 26.2-.4-.6-.8 are also seen around here... i've never seen a 25.6 or 25.8, ever.