The physical resolution of the screen is 2880x1800.

The scaled resolution modes work by rendering at 2x then downsampling. So in the mode you quote it is actually rendering at 3840x2400 (!!) then downsampling, but there's still only 2880x1800 physical pixels.

that's with scaling. the actual pixel level resolution of the retina mac is two times 1440x900 so 2880x1800