Why different available updates in Drush to admin/reports/updates?
Think it was using an old (i.e. Debian Squeeze's) version of Drush. Solution:

Add backports to sources.list

#
# squeeze backports
#
deb http://backports.debian.org/debian-backports squeeze-backports main contrib non-free


Then apt-get install drush=4.5-2~bpo60+1 

Your version number may vary. Check apt-cache policy drush to see what versions are available.