It's likely the tyre compound,
I know that manufacturer's stated widths are not always true to real life but I just want to know why the size stated differs between the regular (700c x ... ) and the etro (622 x ...) sizes.
Interesting comment from Bicycler about half way down this page with the table posted, about conversions from imperial.
https://forum.cyclinguk.org/viewtopic.php?t=100928
@Mukashi started
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I know that manufacturer's stated widths are not always true to real life but I just want to know why the size stated differs between the regular (700c x ... ) and the etro (622 x ...) sizes.