When you take a screenshot of your desktop, your image is of a different size than your display resolution because OS X is rendering a desktop that is effectively larger than your display's pixel density.
OS X saves screenshots of your desktop at the pre-rendered resolution (3,360 x 1,800) rather than the final outputted resolution of your display hardware.
You can't fix this. The sum of pixels is real and wanted as it is. Especially when you use screenshots for print publishing. You need to know the pixel density of your device that made the screenshot and then resize that picture by the ratio. For postproduction the sum of the pixels can't be high enough but for online use it has only an advantage if the original size and density is kept. So it still looks sharp on hd mobile devices. Therefore my suggestion for XF.