Because it all depends on what packages are upgraded.
Doing apt-get upgrade causes apt to upgrade almost everything that it can, with limitations in the case of complex dependency manipulations.
apt-get dist-upgrade is even more aggressive and will perform wide-scale package removals and replacements when these are required by major changes to an application package.

The above upgrade methods are designed for, and work with, 'pure' Debian installations.

The safest approach, as you are using Knoppix, is to apt-get install *only* the application packages that you want to install (or maybe upgrade).
Usually (as demonstrated by the many successful remasterings) this is OK, but there are no guarantees - you just *might* be unlucky in the application that you want to install - it may come down to trial and error.

In any case, before doing apt-get install whatever.deb do
apt-get -s install whatever.deb
which will only simulate the installation and will report what it would do.