Reddit never removes people from the subscriber list AFAIK. So over time, the subscriber count becomes extremely unrealistic. It might claim there's 500 people, but if the sub was created years ago, many of those 500 people probably are inactive. And god knows how many bots might subscribe to a sub for some reason or another (bots obviously don't need to subscribe, but I'm sure many do, since otherwise anti bot measures could notice that they never subscribe to anything). Reddit really should show active subscribers in the past month only.

Lemmy is just so new that if a community has 500 subscribers, that's probably pretty close to the monthly active figure (though with the exception that quite a lot of people have multiple Lemmy accounts because there's been constant reasons to switch instances).

Though also, if you see a Reddit sub with only 500 people, you know it's dead and you should look for a different sub to post in. On Lemmy, 500 isn't utterly awful and also many front-ends only show numbers for your instance, so a community with 500 subs might be a decently sized community (though who can tell?).