700c was around long before 27". As I recall, although the size standard was quickly adopted by other bike makers, 27" wheels and tires represented the last of the proprietary sizes introduced by a single bike manufacturer---Raleigh, I think, probably in the late 1940s. Schwinn was big on Schwinn-only sizes, too.

For more wheel size confusion, try installing a 24 x 1 3/4 tire on a 24 x 1.75 rim. Completely incompatible. 24 x 1 3/4 is a Schwinn-only size.