Tire sizes should be thought of as names not measurements.

Most 26X1-3/8 wheels take a 590mm tire, also called 650A or EA3.

Schwinns marked 26X1-3/8 sometimes came with a wheel which needed 597mm tires. I think those were proprietary to Schwinn and are marked S-6 instead of EA3.

26" MTBs take a 559mm.

I think there are at least eight sizes called "26 by something".