DisplaySize width height
This optional entry gives the width and height, in millimetres, of the
picture area of the monitor. If given this is used to calculate the horizontal
and vertical pitch (DPI) of the screen.
Identifier "My Monitor"
HorizSync 31.5 - 48.5
DisplaySize 351 229
dimensions: 1024x768 pixels (351x229 millimeters)
resolution: 74x85 dots per inch
Gimp and firefox pick-up the incorrect dpi (gimp reports system resolution 74x85).
I was wrong, it's calculating dpy correctly.