12 fret join equals short scale length???

[John Parchem]

There have been a bunch of times I heard knowledgeable guitar people, players and guitar store owners explain a big difference with a 12 fret join vs a 14 fret join is a shorter scale length. When I first heard it I assumed the presenter was somewhat ignorant about the differences as I make long scale 12 fret join guitars. I assume the 12 fret join = short scale length comes from a history of one or two guitar manufacturers product lines.

Recently when drawing plans for a nylon string classical with a 14 fret join, one of the changes I made was to go from a 650 mm to a 660 mm scale length as a way to get the bridge close to where I wanted it in the lower bout. That got me thinking maybe that is how the correlation between the number of frets above the body and scale length originally started. For example Martin changing a 12 fret join guitar model to a 14 fret model also changed the scale length.

Does anyone know the origination of the assumed correlation between fret join and scale length?