Why does Ubuntu use gdm 2-20 when GNOME was releade with 2-24

[Kenneth Nielsen]

Why does Ubuntu use gdm 2-20 when GNOME was released with 2-24. I only
ask because we actually used quite some time on implementing
conventions about the different computer states (suspend, hibernate,
sleep etc.) in all the GNOME packages (the current versions of them of
course) including gdm, but that isn't really much good if they aren't
used ..!

Regards Kenneth Nielsen