It would be much less confusing if the actual measurement were specified. There are at least 5 different rim sizes that are all called 26":
559mm (the mountain bike size), 571mm, 584mm, 597mm, and 599mm. I first encountered the problem almost 50 years ago when trying to buy a replacement tire for my Schwinn 3-speed. The tires on it were labeled 26 x 1 3/4" so I went to a store that sold bike tires and they had some labeled 26 x 1.75". Knowing that 1 3/4 = 1.75 I figured that should fit and brought one home. Spent the next couple hours trying to get the tire on the rim, but neither I nor my father had any luck so we finally went back to the store. There we learned that the Schwinn 26" rims were just a little bigger than the 26" rims used by most other bike makers at the time and that we'd have to go to the Schwinn dealer for tires that fit.

Similar situation with 27" and 700c - no way to tell from the names what the actual sizes are or by how much they differ. If we switched to using the actual measurement we'd refer to 27" rims as 630mm and 700c ones as 622mm which would make the situation much clearer. On top of that, the 622mm size is sometimes called 28" and in the MTB world is called 29" despite being a slightly smaller rim than those called 27".