Why does Update Manager download full packages which already installed? For example, when there is an update for Firefox or Virtualbox or any other software, the Update Manager shows that it's downloading it from scratch.
The package--that is, a single
This is not unique to Ubuntu, by the way. Most operating systems, Unix-like or not, usually download significant information during updates/upgrades that is already present on the system.
So, that's the reason. If you're trying to request a feature that this be changed, Ask Ubuntu is not the place for that. For this, try Brainstorm (please search first, as somone may have already posted the idea).
Jorge Castro has commented about a particular implementation of this feature. This may or may not be what you're thinking of (and is not necessarily the only way to achieve this), but is likely the most fruitful path toward the feature you're interested in.