Quite commonly bikes are not measured properly. Sounds like this is an ebay type situation, rather than a catalog one. Cats are really bad.

Sloping top tube to some degree would make sense, even if they look relatively level. Another possibility is that the larger frame is not it's full size just because it is called a 23 doesn't mean it is. There can be a 1.5 difference in BB heights. Normally there is a preferred height for a given service, but I have seen that much variation. I remember this one aluminum touring frame that was so high it could have run 26" wheels as well as 700c with a few tweaks.