I’ve also wondered how these stats were derived. I assume ‘users’ is actually instances of Firefox - e.g. if, as was recently the case, I have release, beta, and nightly on both a desktop and Android then that’s 6 users.
Are updates perhaps counted as downloads? Does syncing have any odd effects?
One of my addons has 430 downloads and 12 users - 2 of which are me. My guess is that these downloads were bots or Mozilla processes but it’d be nice to know. I cited it in a bug report so that will account for a few.
FYI Here’s the change erosman referred to: https://github.com/mozilla/addons-server/issues/3632