Why do I sometimes get a gdm similar to gnome 2's?

[Marco Scannadinari]

Sometimes, usually when updates are installed, after booting, I get a
gdm screen similar to this -
http://lippolweblog.files.wordpress.com/2009/09/gdm.png. It uses the
same animations as the new black gdm, but it just has the plain white
layout as of the old gdm 2.x.
Has anyone experienced this? What is the cause?
-- 
Marco Scannadinari <marco at scannadinari.co.uk>