There are quite a few more than two sizes of 26" tires.

See the Sheldon Brown site.
https://www.sheldonbrown.com/tire-sizing.html

Initially the tires were essentially sized by the outer tire diameter. So, a 26x1 tire and a 26x2 tire would have about the same outer dimension which required different rim sizes. Plus tires and wheels were made around the world, with each manufacturer doing things a little different. But it became a mess until a few rim sizes were mostly standardized.

Then what was done was putting larger tires on the mid-sized rims (650b), so they really aren't 26" anymore.

Anyway, there is greater standardization now, but the vintage bikes will remain in use for quite some time and one just has to pay attention.