The confusion comes the "p" names generally being used to refer to fullscreen 16:9 resolutions. 1080p = 1920x1080, 900p = 1600x900, 720p = 1280x720, so on and so forth.
The Order runs at 1920x800, a 2.4:1 aspect ratio with black bars on the top and bottom of the screen. (It does have progressive scan and 800 lines of resolution, but that's not necessarily "800p" in the common usage.) This is better for the overall clarity of the game since there's no blur introduced by stretching the image, like there would be for