Jesus christ the replies in this thread are ridiculous.
The Retina MacBook Pro 15", displays EVERYTHING at 2880x1800, because that is the resolution of the screen. It's always 2880x1800.
Example:
http://b2b.cbsimg.net/blogs/win8_default_scale.png
The problem is that running any operating system whether is be windows or OS X at 2880x1800 is that every single icon and app is incredibly tiny. You cannot use the operating system at this resolution on a 15" screen. It would be ridiculous.
So what Apple does is they enlarge the interface, example with windows:
http://b2b.cbsimg.net/blogs/win8_custom_scaling.png
This is called "scaling". The purpose of this is to make the interface operate as if you had a "normal" resolution, so instead of clicking on icons the size of ants, you're clicking on icons that are sized normally.
This is confusing because instead of calling it "2x Interface Scaling", Apple likes to explain this by basically saying "It's going to look like an imaginary computer with the same screen size that has a resolution of 1680x900 with scaling set to 1 ". Except they don't tell you that this is "imaginary" and "like" and "default scaling set to 1", they just say "scaled resolution: 1680x900", which isn't very informative if you don't know what it means.
Hope that helps.