package management - Choice of repositories and link with Software Updater warning message "Not all updates can be installed"?

I am on Xubuntu 14.04. I don't understand the cause of the following warning message I get.

1. When I check only the Universe repository, Software Updater tells me that my system is up to date.




2. When I check the following repositories in Software Updater:

   
   
   
   
   
   • Main
   
   
   • Universe
   
   
   
   • Restricted
   
   
   
   
   

   I get a list of package to update. Up to now, everything is sound.




3. Now, if I check only the following repositories:

   
   
   
   
   
   • Main
   
   
   • Restricted
   
   
   
   
   
   

   I get the message "Not all updates can be installed".

   
   
   

   If I click Continue, I get a list of packages to update but a number of them are unchecked and cannot be selected:

   
   
   
   
   
   • Complete Generic Linux kernel and headers
   
   
   • Generic Linux kernel headers
   
   
   • Generic Linux kernel image
   
   
   • GTK+ graphical user interface library
   
   
   
   • Programs for the GTK+ graphical user interface library
   
   

Can it be that these packages are in the Universe repository and that I get the warning because I did not check this repository? But then, how comes I am not offered to upgrade them when I check just Universe?

I did a sudo apt-get install -f and it says:

0 upgraded, 0 newly installed, 0 to remove and 44 not upgraded

How can I make sense of this behavior? I know I could just check all three repositories: Main, Restricted and Universe, and just proceed to update; but I would like to get a deeper understanding.

PS: I have 'Important security updates' and 'Recommended updates' checked and I did not check any of the Other Software repositories.