MacBook Pro screen resolution confiusion
Many many OS X versions ago, Apple rolled out "Resolution Independent UI" in order to ensure that UI elements were rendered consistently regardless of the DPI of the display. Prior to this, all displays were assumed to operate at 72 pixels per inch (ppi). Prior to that, if you wanted the main Mac menu bar to be 3/8" high, you would simply multiply 3/8 by 72 to set how tall the element is.

Why do I bring this up? Because with this foundation in place, Apple rolled out retina displays with DPIs approaching 300 ppi. Your display is one of these and indeed has a Fixed dpi of 2560x1600 (per specs). The underlying resolution independence ensures that the UI elements display at a consistent size across resolutions, screen sizes, and pixel densities.

In summary, from my understanding:


Your Screen's resolution is fixed at 2560x1600px (see note below)
Your "virtual" resolution is set in displays preferences, and you have it set to "look like" 1400x900
"Full Screen" screenshots (⌘-⇪-3) appear to capture in "native" display resolution. (see note below)
"Marquee" screenshots (⌘-⇪-4) appear to capture in "virtual" display resolution.


note: I cannot say where the 2880x1800 resolution comes from in your screenshot