When I run xdpyinfo on my ASUS UX305, it says:
dimensions: 1920x1080 pixels (508x286 millimeters)
resolution: 96x96 dots per inch
Since the dimensions are obviously wrong, I set up my Xorg config to give the correct size (294x166).
The resulting Xorg logfile shows 165x165 DPI, but xdypinfo still says 96x96.