21:9 aspect ratio

The label is a rounding, and an inconsistent one. A panel at 2560 by 1080 works out at 64:27, which is 2.3704, and one at 3440 by 1440 works out at 43:18, which is 2.3889. The nominal 21:9 is 2.3333, so both real shapes are wider than the name, and they are not the same as each other either.

There is a logic to 64:27 even though it is never printed on a box. It is 4:3 cubed, which means it tiles cleanly against both 4:3 and 16:9 in a broadcast chain, and it was adopted as a standard for exactly that reason. 43:18 has no such elegance behind it; it is simply what you get when you extend a 1440 pixel tall panel to 3440 pixels wide.

The practical effect is that designing to the label produces artwork that does not fit. A wallpaper built at a true 21:9 and shown on a 3440 by 1440 panel is either stretched or shows a thin bar, and the error is small enough to look like a rendering fault rather than a ratio mistake.

The fix is to work in pixels for this class of display and let the calculator confirm the shape. Enter the real panel dimensions, read the simplified ratio it produces, and use that rather than the number on the marketing material.