Differences to other sites is probably due to the set of instances checked
https://fedidb.com/software/lemmy?search=lemmy.world&stats=1
Fedidb doesn't seem to include lemmy.world, which seems like a substantial omission, as I believe that it's the largest Lemmy instance.
EDIT: And my own home instance, lemmy.today, also doesn't seem to be on there:
https://fedidb.com/software/lemmy?search=lemmy.today
The largest user count server that they seem to query is lemmy.ml:
https://fedidb.com/software/lemmy?search=lemmy.ml
EDIT2: Your home instance, kbin.earth, does seem to be on there: