Comment 2 for bug 734026

Technically you are "upgrading" your system. An update just checks the repos in your sources.list for newer versions of packages. An upgrade actually installs the latest packages that update has found. Here is the man page for upgrade in apt-get:

--upgrade--
upgrade is used to install the newest versions of all packages
currently installed on the system from the sources enumerated in
/etc/apt/sources.list. Packages currently installed with new
versions available are retrieved and upgraded; under no
circumstances are currently installed packages removed, or packages
not already installed retrieved and installed. New versions of
currently installed packages that cannot be upgraded without
changing the install status of another package will be left at
their current version. An update must be performed first so that
apt-get knows that new versions of packages are available.