The reason for the discrepancy between the first and second of these sets of figures is the space utilized by the operating system control bars which is therefore unavailable to the browser.
There are two reasons for the discrepancy between the second and third sets of figures. One of these is that you may not have the browser window maximized on your screen. The second is that the browser control bars themselves take up some space on the screen making less space available within the window.
If you have selected your actual screen resolution on the previous screen and this browser window does not fill all of the available space on your screen then there is a different browser where the default configuration would only give the viewing area shown when displayed at this resolution.