I hate it when people say "My screen has 1920×1080 resolution.".
No it doesn't. There is a difference between media/graphics card resolution and screen resolution. The movie you play on the screen can be 1920×1080 or the video mode can be 800×600. You could use the number of pixels only to compare screens of the same physical size, otherwise screen resolution is measured in pixels per inch.
For instance, an iPhone 5 with a 1136×640 4" screen has about three times the resolution than a 1920×1080 21" screen. Fewer pixels, yes, but higher resolution: 326 PPI vs 110 PPI.