auto resolution = "Original" not max resolution?

[Kono14]

I set my auto-resolution setting to "Orignial" and thought this setting would always give me the highest available resolution. My screen is 1080p, but I like to watch in 1440p or 4k if available. But "Original" setting gives me 1080p@60fps on a 1440@60fps video. Why?

Maybe I understand the "Original" setting wrong?
And why is there no "4k" setting instead of "Original"?