The calculation includes all published versions of the browser, starting with the first release in 2004. If a user downloads multiple copies for different computers, all of them taken into account. If the user manually downloads from the Web site to update the existing version, the counter is also increasing. But the automatic update does not affect the number of downloads.
Thus, far from the fact that the web browser uses a billion people. Nevertheless, apparently gaining popularity of the application, the former did only a few years ago and today is directly competing with Internet Explorer, by default enabled in Windows.