If you start Synaptic Package Management you can set the "Package upgrade behavior (default distribution)" on the "Distribution" tab of the "Preferences" window in the "Settings" menu. The default is "Always prefer the highest version". This means you will almost always get packages of the "unstable" ("sid") or "testing" ("wheezy") distribution if you decide to upgrade or install new packages.

On the other hand the program apt-get uses the configuration file /etc/apt/apt.conf.d/00knoppix with the first line reading
Code:
APT::Default-Release "squeeze";
At the moment the distribution "squeeze" is a synonym for "stable", but in the future "stable" could be today's "testing" distribution. Therefore it is better to use the distribution's real name "squeeze" instead of "stable".

Sometimes installing new packages conflicts with the version of existing packages. Then the package management program calculates a solution based on your default distribution and the packages to be installed. Very often you get a "solution" that implies the removal of packages which are important for you. Since the two package management programs use different distributions as default it is obvious that you will get different solutions if you want to upgrade or install new packages.

If you decide to use apt-get you have the possibility to specify precisely that you like to install a package from a certain distribution by issuing
Code:
sudo apt-get libreoffice-l10n-en-gb/testing
for example. There is also the possibility to overwrite the default distribution in the command line
Code:
sudo apt-get -t testing install libreoffice-l10n-en-gb
Then apt-get prefers packages from the "testing" distribution.

If apt-get tells me that a package cannot be installed because another package "is not going to be installed" then I try to append the missing package to the list of packages I like to install. In your case I would try the command
Code:
sudo apt-get install <package you like to install> python-pyorbit python-gnome2
with <package you like to install> substituted by the real name of that package.

You can use the program apt-cache to learn more about your packages. Example:
Code:
apt-cache policy libreoffice-l10n-en-gb
Using Synaptic is comfortable, but apt-get gives you better control over the whole installation procedure.

I hope this helps.