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"safe-upgrade:   Upgrades installed packages to their most recent
version. Installed packages will not be removed unless they are
unused. Packages which are not currently installed may be installed to
resolve dependencies unless the --no-new-installs command-line option
is supplied.

It is sometimes necessary to remove one package in order to upgrade
another; this command is not able to upgrade packages in such
situations. Use the full-upgrade command to upgrade as many packages
as possible."

"full-upgrade:   Upgrades installed packages to their most recent
version, removing or installing packages as necessary. This command is
less conservative than safe-upgrade and thus more likely to perform
unwanted actions. However, it is capable of upgrading packages that
safe-upgrade cannot upgrade."

So, the reason it is not upgrading those packages is because it will
have to remove some other packages.

If you want to see the exact reason, running:
    aptitude why-not linux-generic
should tell you why it won't install that package.

(And this is why aptitude is recommended over apt-get too.)