I just ran into this problem and I was wondering if anyone knew the lemmy software well enough to answer this. I'm seeing different stats on communities depending on which instance I view it from, and I don't know what the true statistics are.
Using a community I moderate ( /c/maliciouscompliance@lemmy.world ) as an example:
If i view the sidebar from its home instance: https://lemmy.world/c/maliciouscompliance
It says 91 users / day, 143 users / week, 2.19K subscribers
But if I view it from another instance:
https://lemmy.ml/c/maliciouscompliance@lemmy.world
It says 42 users / day, 85 users / week, 45 subscribers.
I'm assuming the user count is based on that specific instance, and subscriber count when viewing from another instance is based on subscribers from that instance only (so 45 subscribers using lemmy.ml accounts). But is the subscriber count from the "home" instance the true aggregation of all subscribers across the fediverse, or subscribers from that instance? And if the latter, is there a way to know the true subscriber count for communities across the fediverse?
As far as I can tell, the subscriber count on the home instance appears to be the total across instances. As a test, I created a community on my instance(custom w/ 1 user) and my friend subscribed to it. The count displayed was 2, which seems to indicate that it tracks everyone who has subscribed to it, whereas the other instances don't have access to detailed information about the community and can't accurately determine that.