I think they count every download of every package, every version, every time. It's not the number of unique users or even packages.
If you install 3 apps you might need to download 3 versions of graphics driver, 3 versions of desktop environment libraries and so on, It won't count as one user installing 3 apps, it will show up as 10 -20 downloads. And that's just the initial install, every time you update them it counts another 10-20.