You can use System Information to check, but I think you'll find that the Mac is running in a Retina "UI looks like 1470x956" mode. It will be drawing on an internal 2940x1912 pixel canvas, and downsizing the image to fit on the actual 2560x1664 pixel screen.
You can get to System Information… by holding Option while clicking on the Apple () menu.
If it was doing perfect 2x scaling, the "UI looks like" resolution would be 1280x832 pixels. That would map to a 2560x1664 pixel canvas that exactly matched the resolution of the 2560x1664 pixel LCD screen.
Either way, the idea behind the Retina modes is to keep text and objects large enough to read (thus the "UI looks like" resolution), while allowing Retina-aware applications (virtually all of them, these days) to draw text, and fill in photo areas, in greater detail.
It sounds like Apple set the default for your machine a notch towards cramming more, but smaller, stuff onto the screen.
You could probably find third-party apps that would let you set your screen to a 2940x1912 non-Retina mode, but then you'd find that while you could cram a ton of text and windows onto the screen, everything would be tiny and difficult or impossible to read.