I suppose a scale could have just one set of ratios, but the ones all over my desk are triangular and have many ratios, in metric and in Imperial.
Taking info off drawings, as I often do, makes it obvious that the scale of drawings have lost a lot of accuracy over the years.
I have backwards traced why some of that happens. In the old days, a drawing was made to scale then copied 1 : 1 in a blue print machine. The scale remained intact. Now, with CAD generated drawings, the relative scale remains intact, but the absolute scale can vary quite a bit depending on the digitized version of the originals. I have a client who has his raster density set for his plotter, and he sends his files to me via e-mail where mine has a HP engine and his a Canon (300 basic DPI and multiples thereof vs 360 DPI) and sure-as-shit, his stuff is always off by 20%. My large format (bigger than C) plotter print shop always asks for a 50cm reference line so that my drawings are at least true to my manual scales. Architects have little problem with things like that, but the rogue "kitchen designer" who has bought a CAD package for a few hundred dollars are not to be trusted when it comes to scale.
And all along I thought that scale was what the union said what I should get paid for doing a gig. :)