I think it's quite complicated though.
My eldest has 1+3/8 tyres and rims. BUT the tyres are less than 1+3/8" wide, and the rims themselves are less than an inch wide with the gap between the tyre beads only being 16mm, ie a lot less than 1 inch. In fact his 1+3/8 rims look the same width as the 1+1/8 rims my little bro raced on at the same age. How does that work