As Jim said, I'll bet it's a hair over 26.0 mm. That "hair" is the important part and it's tough to determine with a good caliper, let alone a cheap plastic one.
Your local bike shop may have a seat post diameter measuring rod, and it might be best to see if they do. One thing, before you measure it again, take some 400 grit sandpaper and clean the paint out of that seat tube. And then lightly grease the inside of the tube. You'll be glad you did.