In the early 1980's, the ANSI standard for projection apertures became .825" wide (20.96mm) for scope and 1.85. Some charts just round 20.96mm to 21mm.
.825" was already the standard width for Academy 1.37 and 1.85, 1.66, etc. so it was just the standard for the anamorphic aperture that changed to match the rest.
Current standard for an anamorphic aperture for a projector is .825" x .690" (1.1956521 : 1, which doubled becomes 2.3913 : 1.)
Now the same chart in the ASC Manual converts that to metric as 20.96mm x 17.53mm.
Before the early 1980's, the standard for an anamorphic projector gate was .838" x .700" (1.1971428 : 1, which doubled became 2.3942856 : 1.) This was the new standard put into place in the early 1970's.
In 1957, the standard was set as .839" × .715" (1.1734265 : 1, which doubled became 2.346853 : 1.) You can see how the height of the gate has been shrinking over the decades to hide any errors in making negative splices. The space between the frames in CinemaScope was so small that you had to use a special negative splicer to be more precise and any bad splices would show up on screen as a flash of light on the top or bottom of the image depending on the projection. So a thicker frame line due to a shorter gate hid those problems better.
Today, for a 2K "scope" DCP, the standard is 2048 × 858 pixels (2.3869463 : 1.)
Now again, I'm talking about projection apertures and aspect ratios. The camera gate is often larger than the projector aperture -- what matters more is what your groundglass frame lines tell you is the area you are composing within.
Panavision, for example, lists in the ASC Manual that their 4-perf 35mm anamorphic camera aperture is .866" x .732" (22mm x 18.59mm) but the truth is that most of their cameras will actually expose Full Aperture, it's just that your lens is centered for the sound aperture and you are composing for the area to be extracted for projection. But even the standards for the area scanned is slightly different again, which is why sometimes it is simpler to just talk about the area that gets used for projection.
What matters the most is that the squeeze/desqueeze is a standard 2X amount. The other difference is mainly in just how much of the negative is trimmed, which affects the final aspect ratio.