The main thing about the games on Desura is that most of them are installed in your /path/to/desura/common directory, which is somewhere in your /home folder. Games on Synaptic or the Software Center are installed in your (possibly) /usr/share/games folder, which is why most all games on Desura don't need a password to be installed.
I really hate it when games install in "/usr/share/games" and ask for password. My root partition isn't too big and games usually require quite a bit of space.
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