Why does a 4K display show 200% or an 8K rendering resolution?

The resolution slider’s ordinary percentage compares the logical desktop size with its default size. It is not the percentage of physical pixels in the panel, and it does not by itself say whether HiDPI is enabled.

The macOS scaling explanation distinguishes desktop size, rendering pixels and physical output.

For example, if a display’s default logical desktop is 1920 × 1080, selecting 3840 × 2160 makes its width twice the default and can show 200%. Both a HiDPI and a LoDPI mode can have those same logical dimensions, so turning HiDPI off need not change that percentage. The default depends on the display and its configuration. Use the default shown for your own display rather than assuming it is 1920 × 1080.

With two-times HiDPI rendering, a 3840 × 2160 logical desktop uses a 7680 × 4320 backing image before it is fitted to the physical display. In portrait orientation those dimensions are exchanged. A system report of 4320 × 7680 alongside UI Looks like: 2160 × 3840 therefore describes different stages of rendering. It does not establish that the panel is physically 8K.

Check the selected mode’s HiDPI state and the monitor’s actual incoming signal separately. See physical output versus desktop size. Comparing screenshots or log dimensions alone can confuse a large rendering surface with the signal sent over the cable.

The slider also has an optional magnification-style presentation, which reverses the interpretation so larger values mean larger interface elements. See resolution slider settings before comparing percentage labels between setups. Editing the default logical size can change the baseline. It does not change the panel’s physical pixel count.