They use "effective pixels", just like CSS pixels are not actual pixels:

> Because of how the scaling system works, when you design your UWP app, you're designing in effective pixels, not actual physical pixels. Effective pixels (epx) are a virtual unit of measurement, and they're used to express layout dimensions and spacing, independent of screen density. (In our guidelines, epx, ep, and px are used interchangeably.)

https://docs.microsoft.com/en-us/windows/apps/design/layout/...

Also, Windows scales the pixel size when you use a legacy application (non DPI aware) on high DPI displays.