3150x2210 is sort of a normal resolution for retina displays. It's close to a native panel resolution on iOS, but they do this dumb fractional scaling thing because it was too hard to backport support for high DPI displays to MacOS X. Anyway, that resolution is so unreadable on current macos that they hide it behind a "show all resolutions" toggle. The default is 50% that (1/4 as many pixels), and they only let you go up to about 60-66% of native resolution unless you click the override.

So, the screenshot is probably a semi-upscaled image of a ~ 1920x1200 desktop.