Hi,
as the manpage of apt-get tells:
[...]
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.
dist-upgrade
dist-upgrade in addition to performing the function of upgrade, also intelligently handles changing dependencies with new versions of packages;
apt-get has a "smart" conflict resolution system, and it will attempt to upgrade the most important packages at the expense of less important ones
if necessary. The dist-upgrade command may therefore remove some packages. The /etc/apt/sources.list file contains a list of locations from which
to retrieve desired package files. See also apt_preferences(5) for a mechanism for overriding the general settings for individual packages.
[...]
I have marked the - in my opinion - important and interesting sentence inside the "upgrade" part with two stars, which should be applying here. I hope this helps.