This follows your scaling settings. It reports the resolution of the framebuffer (the image as drawn by the OS before being sent to the display) rather than the resolution of the display.
I think it's a bit misleading that they did this right under the "Built-in Retina Display" title, next to the physical screen size. It might have been a mistake.
If you want information on the display hardware, go to About this Mac->More Info...->System Report...->Graphics/Displays