There are several concepts to clarify as to the retina resolutions, the first thing is that the concept of "resolution" is not the same as that worked up before the retina screens.
- I do not speak of a single resolution, but two: The resolution at which the graphics card is sending images to the screen (previously only talked of this concept), and the resolution that detect applications that are no longer pixels but simply points, this new resolution is the internal resolution.
- Your graphics card is ALWAYS sending the images to the screen resolution (2880x1800), when you select options in 1440x900're actually saying just what the resolution will "see" the apps and the system (internal resolution). In fact each point of the internal resolution, corresponding to 4 pixels in the display. Before the retina screens that simply did not exist, and if you changed the resolution then the graph would send exactly the pixels that your you told.
Once explained above, the answer to your question is not just what you're looking at 1080p, but when you put it to full screen the system is scaling the video to fill the entire screen points, ie this laptop has as much resolution to see all the pixels of 1080p video even you lack maximize the window.