Macs ship with the display set at a certain resolution, and Apple defines this in the technical specifications for each model. But with Retina displays, these numbers can get confusing: there is the display’s resolution and the “looks like” resolution used on the Mac. Resolutions on Retina Macs look like half the actual number of pixels measured vertically and horizontally because of “pixel doubling.”
Read the rest of the article on Macworld.