Why does Xvfb not support the backing store?

I am running an X server (X.Org v. 6.8.1 on GNU/Linux Fedora Core 3,
2.6.10-1.770_FC3smp, i686) with backing store capability turned on
(the "+bs" option used when starting the server).  Moreover, I am
forcing the default backing store of all windows to be WhenMapped (the
"-wm" option used when starting the server).  The server is run with
this command:

/usr/X11R6/bin/X :0 -audit 0 +bs -wm -auth /var/gdm/:0.Xauth vt7

I started several applications, whose windows overlapped.  Then I got
the window ID's of these applications with:

xwininfo -root -children

Finally, I used the xwd command (there I passed the ID's) to save the
images of the windows.  The image of every window contained no
overlappings, just as I wanted.  Conclusion: the backing store is
working.

Up to this point things work as expected, but they go wrong when I try
to repeat the same process with Xvfb, i.e. the X virtual frame buffer.
So I run Xvfb with:

/usr/X11R6/bin/Xvfb :1 -audit 0 +bs -wm -auth /var/gdm/:0.Xauth

Next, I run the applications and get the ID's.  Finally, I get the
images with the xwd command.  Result: the window images have
overlappings.  Conclusion: the backing store ain't working!

QUESTION: why does Xvfb not use the backing store even though I
request it?

Thanks for your time!


Best,
Irek