The mathematical aspect ratio of a display gets "corrected" to the more familiar ratios that people are used to. This should be done even if the screen is in a different orientation. In particular, the Steam Deck uses and 800×1280 display, which is then rotated to act as a 1280×800 display. Most people would expect this to be described as a 16:10 (or 10:16) display. Instead it is described as a 5:8 display.
A possibly relevant merge request was started @ https://invent.kde.org/plasma/kscreen/-/merge_requests/130
Git commit 072f7d26506785319dfc0a06c534040d8d8f2430 by Aleix Pol. Committed on 13/09/2022 at 23:54. Pushed by apol into branch 'master'. kcm: When simplifying ratios, take into account that they could be rotated The resolution could be 800x1280 instead of 1280x800 and we still want to call them 10:16 rather than 5:8. M +4 -4 kcm/output_model.cpp https://invent.kde.org/plasma/kscreen/commit/072f7d26506785319dfc0a06c534040d8d8f2430