Difference between M-x and call-interactively

[Jambunathan K <kjambunathan@gmail.com>]

Apropos bug:
http://debbugs.gnu.org/cgi/bugreport.cgi?bug=12218

The bug pertains to focus shifting away from ediff control frame after
pressing `n'.  This problematic behaviour happens with vc-ediff and not
with ediff-files.  The current findings is that it happens just on my
Xfce machine. (IIRC, I have seen this on Windows XP.  I don't have the
Windows machine with me now, so I can't verify.)

I was trying to narrow this issue further and ended up observing
whatever the subject claims.

1. C-x C-f some-file
2. C-x C-w some-file-1
3. Make some random changes some-file-1
4. M-x ediff-buffers RET (some-file and some-file-1).  Press n in
control panel.  I note that the focus stays within the control panel.

Now instead of step 4 above, if I C-x C-e below form in scratch buffer

  (call-interactively 'ediff-buffers)

I see that the focus shifts away from the control panel.

In summary, M-x ediff-buffers gives correct behaviour.  But C-x C-e of
the interactive call yields buggy behaviour.

Any ideas on what could cause this behaviour?

ps: I thought I would move the discussion here so as to get more
eyeballs.
--