This morning my Retina MacBook wouldn’t display over 1080p resolution to my Dell U2713HM. Normally, it displays at 2560×1440 via a miniDisplayPort to DisplayPort cable. The mac would not show anything above 1080p even by forcing all resolutions by holding the Shift key while selecting “Scaled” in the Displays preferences. After hours of attempting to reset the resolution by restarting, resetting the display, unplugging and replugging the cable from the mac and the monitor, I switched my Thunderbolt to Ethernet adapter with my DisplayPort cable and VIOLA! Normal function has returned.
Essence of the story: switch ports before any other troubleshooting method for external display resolution on Macbook Retinas.