I guess camera manufacturers are not that much different from lumber yards that sell "2x4 lumber" that is distinctly not 2x4 inch. Not by a long shot
The 2"x4" sizing actually relates historically to the wood in its rough form before it was milled to its final size.
Yeah, I understand, the frame spacing on my GW690 will depend on the winding system. I believe simpler advance systems will not be able to adjust for start-of-roll vs end-of-roll discrepancies in spool diameter. My Koni Omegaflex has a fancy system that adapts the wind to the shot number, making the frame spacing equidistant.
As indicated above, the different spacing is probably more related to the need to use a smaller film gate in some mechanisms in order to ensure film flatness.
Of the various cameras I have, the film gate sizes vary slightly between cameras, but the spacing is in most cases equal and consistent within each individual camera, compared to the frame numbering. The only thing that varies in most cases is the space between frames.
A piece of film that is nominally 6x6 or 6x7 or 6x8 or 6x9 cm is quite difficult to keep tensioned and flat to the necessary tolerance unless it can be held against a fair bit of metal in front and behind. Different film paths influence how much of the camera needs to be pressed into that film from both sides in order to accomplish that.
The other influences that need to be taken into account include:
1) the fact that 120 film was also used to make projectable slides, and slide mounts always intrude into the film; and
2) the commercial photo-finishing industry in various parts of the world tended to use slightly different equipment, with different negative masks, printing on to different sizes of paper.
You might very well find that back in the times that these cameras were being sold mostly that a machine printer used in a lab in Germany would use negative masks of one size, the labs in the USA and Canada would use printers and masks that were sized slightly differently, and the labs in Japan would use printers and masks that were a third size.
As the big Fuji rangefinders were almost exclusively sold originally into the Japanese market, there is a very large chance that the film gate sizes for them were expressly designed to match the needs of the Japanese photo-finishing market. A really large proportion of what happened with film, cameras and photo-chemicals was designed pre-dominantly with the needs of the photo-finishing industry in mind.
The inclusion of a rough indication of frame size in a camera's model name was never assumed to be anything more than a means to differentiate between models in a line or segments in the marketplace.
The idea of a filed out negative carrier - one that reveals the entire image plus perhaps some of the rebate - is a fairly modern thing. Machine printers and enlarger negative carriers were expected to block a bit of the image area. If the photographer cared about that - and relatively few did - they would go to the effort of using an oversized glass carrier instead.