To calculate the DPI it uses the screen resolution (pixels) and your reported screen size (inches), if either is missing it would fail to work (for now).
I'm not sure if that can be disabled or not or is system dependent. (I'm assuming every system reports it correctly as of now).
If your system reports the resolution correctly here - http://andylangton.co.uk/articles/javascript/browser-screen-... - then it must be some other bug.
To calculate the DPI it uses the screen resolution (pixels) and your reported screen size (inches), if either is missing it would fail to work (for now).