‘Users’ on the addon page is an average over some period. I have addons where that is about half the number using it on some days, which depends on how many machines check for updates in some 24hr period.
It should be reliable now when it’s using the telemetry data (as long as users has telemetry enabled).
I remember when I was working on a new addon for a week and then I’ve released it, it the stats I could see it already has one user for exactly one week! Which is amazing because it means it can track even not released addons! Cool, right?