Don, great question that I have oft pondered myself.
I suspect.........that once mass-production (in the vintage sense) framemakers had a jig for a popular frame, they just kept making them the same way. Of course over time, component choices would change with the fashion and at this point compatability would become an issue. For example, if the maker were to change to a different caliper, they wouldn't change the long-successful frame design, they'd just get the calipers that fit. If the front needed an Acme 500 and the rear an Acme 750, so be it! Same difference if they changed wheel sizes (a la tubular to 27" clincher). I say this particularly with the Ral Int'l in mind. Most of us know this bike started life as a Carlton (thank you) so far back who knows what type of brakes they were running. Didn't matter....long as they fit.
I'm probably wrong but it made sense to me at the time.