Until recently each time you ran "apt-get update" you would download each complete package list from the sources you've got configured - regardless of the fact that very little might have changed since the previous time you did so.

In an attempt to make package updates more dial-up friendly, or easier for other users who are on a low-bandwidth link this system of package downloads was recently augmented by the addition of "package diffs". Rather than downloading the complete package lists upon each update only the difference to the package lists are fetched.

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