I don't use the 6x9 format. I use the 2.25 x 3.25 inch format. Over the years it has becom the norm to speak about various medium formats using rounded centimeter values - 6x9 cm means 2.25 x 3.25 inches even though it is more like 57mm by 83mm or 6x8 cm.
When I was in Germany in the early 80s' I bought a Galvin "6x9" view. It came a with a roll back and some 2 1/4 x 3 1/4 sheet film holders. I went to Munich and bought some "6x9" sheet film. Agfa 100 as I recall.
The film didn't fit my sheet film holders. It turns out that the size of 6x9cm sheet film is 2.5 x 3.5 inches. A ha!
There are two small sheet film standards: Europe 2.5 x 3.5 inches and US 2.25 x 3.25 inches.
2.25 x 3.25 inches is 57.15mm x 82.55mm (6x8?)
2.50 x 3.50 inch film is 63.5 x 88.9 mm (6x9).
Another issue is the actual frame size of 2.25 x 3.25 images. 8 exp 120 roll fim backs have film gate openings that are 2.25 x 3.25. When you use 2.25 x 3.25 sheet film, that image size is smaller due to cropping by the sheet film holder. 2.25 x 3.25 inch sheet film is exactly that size edge to edge. That is why Beseler made two different 2.25 x 3.25 inch negative carriers - one for roll film and one for sheet film (well, they made a third - the universal glass negative carrier but that's a different topic!).
So the term "6x9 CM" developed from 2.5 x 3.5 inch sheet film not from 2.25 x 3.25 sheet film or roll film images.