This is a question we often get, along with a screenshot like this:
So I'd like to explain what we are seeing in this screenshot, and why the Launcher is, in fact, NOT reinstalling the whole game in this case.
What does "patching" or "updating" the game actually mean?
Imagine the game installation as a big fat book of 1200 pages.
The "patch" is a list of corrections or changes to the book. For example:
1. "on page 5, line 20, change the word 'good' to 'well'",
2. "on page 73, after the second paragraph, add the following text: 'blah blah blah'",
3. "on page 135, remove the 3rd paragraph, that starts with 'something something'".
The Launcher downloads the list of changes (in the case of the image above, that list is 435MB of data) and then starts to apply them.
The "install" number shows how far it has progressed in updating.
If we go back to our book example, then at first the launcher will show
Downloaded 3/3 (because we had 3 fixes to apply)
Installed 0/1200 (because it hadn't fixed anything yet)
After applying the first change (change "good" to "well") it will show
Downloaded 3/3
Installed 5/1200 (because we have fixed up to page 5)
After applying the second change, it will show
Downloaded 3/3
Installed 73/1200 (because we have fixed up to page 73)
After applying the last change, it will show
Downloaded 3/3
Installed 135/1200 (because we have fixed up to page 135)
And then it will think for a while because it does all sorts of checks, and then finally jump to "complete".
But really all it did was change those 3 places in the book. It didn't "reinstall" all 1200 pages.
The Launcher does the same thing with its list of updates to the game files. It changes a few bytes here, adds a bunch there, maybe deletes some things, all according to the patch data it downloaded. But the bulk of the 12GB does not get changed, even if it says in its progress display that it's "installing" 12GB.
I hope this makes things a little clearer.