A lot of the users on here just lurk and don't participate in discussions. That's how you get a community with 10 or 20 subscribers and only a couple of active members that actually contribute.
As I've heard it, Lemmy only counts "active users" to be accounts that have commented or posted. Users can visit, browse, subscribe (and i think vote?) But will not be counted in the user total.
There's also the high probability that lemmy is just not tallying them right... not sure how it handles users browsing and interacting from other federated instances tbh
Personally I like that metric. It would almost be nice to have both an "active users" as well as a view/visit count. It's nice knowing how much actual activity a community is getting. It's something that sometimes feels missing from reddit.