Some of the strangeness is probably because, at the minute, the 'active users' list is a mix of instances that count them as people who post, comment or vote (e.g. lemmy.ml), vs. those that count them as people who just post or comment (e.g. lemmy.world).
The rest is probably because, for those that have changed how they count, there's a bit of a ripple effect which'll take a while to dissipate. For example, lemmy.ca upgraded to 0.19.0 overnight, so results from any of it's communities aren't included today, because it would be trying to calculate a change from one measuring system to another (e.g. nostupidquestions went from '103' to '791' active users, but it didn't really, obviously).
However, whilst I'm trying to mitigate the changes from measuring systems, I can't do much about the fact that now including voters has an exponential effect on how many active users a community has. When lemmy.ml upgraded, [email protected]'s active users went from '6' to '23', and I didn't include that change, but there's a been a popular post since then, and as we're now including the voters, the users are at '138'. This is a much bigger change (from '23'), but as far as the new measuring system is concerned, it's a legitimate one.
So, until all the instances are on the same version, and until the change in metric has had a chance to bed in, there'll be some odd-looking results, that I don't think I can do much about.
All I can say is that any linked community does have at least one recent post, so they shouldn't be completely dead. If they appear to be so it's probably for some other mundane reason (blocked users, defeds or poor federation, different languages, etc)
Please use his full name: Anakin Psycho-Cunt (apparently).