I have a mid 2012 MacBook Pro Retina. The native (i.e. maximum supporter built-in display resolution) is, as far as I know, 2880x1800. So far so good. Then I went to resolution settings and slided it all the way right, to the highest offered by the system. No other resolution apps, just the standard OS X setting.
I took a screenshot (shift-command-3). Clicked on it and chose Get Info.
The resolution of the picture was 3588x2276.
The Photoshop shows the same resolution.
How can this be?