You mean 1920 x 1200?
Basically, the native resolution of Retina is 2880 x 1800.
Apple purposefully "locks" OS X to "effective" 1440 x 900, 1680 x 1050 and 1920 x 1200 desktop.
I say "effective" desktop because it's not the true display resolution. The true display resolution is still 2880 x 1800. The "effective" desktops use a scaling factor to scale UI elements from the native 2880 x 1800 resolution to a larger size. That's because the display at 2880 x 1800 resolution makes texts way too tiny for humans to read.
But the scaling factor to developers is still 2.0, so the way Apple does it is... they take the "effective" desktop resolution, multiply it by 2.0, and then scale it down to Retina size.
So 1920 x 1200 "effective" is more like 3840 x 2400 scaled to 2880 x 1800.
That's how Retina looks so sharp.