[CentOS] Inheriting Xauthority

On some of my machines, when I run "su -" in a terminal to get a root
shell, an XAUTHORITY file is automatically generated to give display
access to GUI programs spawned by that shell, i.e.,

       # set | grep XAUTH
       XAUTHORITY=/root/.xauthayZmdH

That file contains a copy of the MIT-MAGIC-COOKIE from my original
X session.

On other machines with the same installed OS (CentOS 6.6) that does
not happen.

What the heck is the difference?  I have done nothing with my own
xauth or xhost to permit or deny this on any of the machines.

-- 
Bob Nichols     "NOSPAM" is really part of my email address.
                 Do NOT delete it.