that's certainly the documented behaviour when the server is started as root. 
see the xorg.conf man page for details.

Well, from my xorg.conf man page, I have to disagree with you.

       When  the  Xorg  server  is started by the "root" user, the config file
       search locations are as follows:

           <cmdline>
           /etc/X11/<cmdline>
           /usr/etc/X11/<cmdline>
           $XORGCONFIG
           /etc/X11/$XORGCONFIG
           /usr/etc/X11/$XORGCONFIG
           $HOME/xorg.conf
           /etc/X11/xorg.conf-4
           /etc/X11/xorg.conf
           /etc/xorg.conf
           /usr/etc/X11/xorg.conf.<hostname>
           /usr/etc/X11/xorg.conf-4
           /usr/etc/X11/xorg.conf
           /usr/lib/X11/xorg.conf.<hostname>
           /usr/lib/X11/xorg.conf-4
           /usr/lib/X11/xorg.conf


The current xorg-server (1.1.0) is also looking in $HOME (for root) before
/etc/X11/xorg.conf, which is - according to the manpage - wrong.

Phil

Maybe it's the

$XORGCONFIG

case, if the X server's current directory is / ?

Go back and read carefully the list you pasted in:

           /usr/etc/X11/$XORGCONFIG
 >>>>>     $HOME/xorg.conf
           /etc/X11/xorg.conf-4
           /etc/X11/xorg.conf