Set custom scaling on Retina MacBook Pro (13inch)
I have a 15" MacBook Pro with Retina Display and by itself, the highest resolution I can set in System Preferences > Hardware > Display > Display > (o) Scaled, is 1920x1200. To get an even higher resolution, e.g 2880x1800 I use a third-party utility named Display Menu. It's available in the App Store for free.