The way Schwalbe uses inches to describe their tires is atrocious.
My understanding is that is how to know the correct tire for a classic English bike. 26 X 1 /3/8" is different from 26 X 1.375. The confirmation is the 37 X 590. Same for Schwinn with 26 S6 rims 26 X 1 3/8 but its 37 X 597