Something I realized: If you visit a community on a different server, it doesn't show all subscribers. Like on Feddit.de, it tells me that there are 993 subscribers for this very community. But if I visit lemmy.ml/c/memes it shows that the community has over 18k+ subscribers. Maybe it only shows the subscribers from your own instance? Like 993 People from Feddit.de are subscribed to this community? No idea.
Since you're registered on lemmy.ml, and thus probably viewing everything through that, I think you only see the subscribers on lemmy.ml. Most people and comms are now at lemmy.world.
I wonder how accurate these statistics are. Total users are over 1mio now, but users that have been active at least once in the last half year are only ~45k. Which is not bad either, but a far cry from 1mio.
I know at least at the start some users were confused by the federation process and created 10+ accounts to "subscribe" to other instances plus there have been bots signing up through smaller instances without capcha or email verification so that might explain the discrepency
The 6-month-active count usually means "This user has been active at least once in the last 6 months" not "This user has been active for over 6 months".
So if you sign up today and spend today on Lemmy, then you have been active at least once in the last 6 months.