Citoyen du monde wrote:
Your standard 27" x 1 1/4" tyre of yore is a 630 ETRTO dimension, the standard tubular size, which has been alternatively called 28", 700c, 27" and perhaps other names too according to distinct naming conventions that have held stead over the years, measures 622 ETRTO. Likewise the modern 29'er tyres for MTB are also 622 ETRTO.
You can't really say that 700c is the standard size for tubulars, since sprint rims can't be measured to the same standard as clincher rims. You measure a clincher rim at the bead seat, a sprint rim doesn't have bead seats. So while they are a very similar size they can't actually be said to be the same.
The fact that tubulars come out a very similar size to 622s is probably a coincidence. The first tubulars in that size apparently predated such skinny tyres being fitted to 622 rims. The 622 clincher rim was probably adopted as a sporting standard in mainland europe because with skinny tyres fitted to them came out a similar size, so they could be fitted to the same frames.