As we know Selenium interacts with browsers and these get
methods will retrieve info related to browsers only. As explained in other answers very clearly, screen resolution and browser are different. The simple example below shows very clearly that the web driver is getting only the browser's dimensions.
WebDriver driver=new FirefoxDriver();
driver.get("http://www.google.com");
System.out.println(driver.manage().window().getSize()); //output: (994, 718)
driver.manage().window().maximize();
System.out.println(driver.manage().window().getSize()); //output: (1382, 744)