The repositories are mirrored and it may take a few hours or days to get all changes copied, especially if there are many.
apt-get update
will show which mirrors are in sync with your installed debs because they will be "hit" and not downloading an update. If they download a new packages list,
apt-get upgrade
may do something, but I normally do not worry and issue both commands routinely. You could use
apt-get update;apt-get upgrade
and likely everything will be automatic.
If there is an extremely urgent update for security, you might include the central repository in your sources. I would not do this routinely because those folks get very busy and we love them.
See
http://www.uk.debian.org/distrib/packages
See
http://packages.ubuntu.com/