The official join-lemmy.org page problematically includes Hexbear, which would account for 1,553 of the difference between it and fediverse.observer. The remaining 4,281 difference seems to be several servers not listed by fediverse.observer.
Those above 50 MAU exclusive to the join-lemmy list are aussie.zone (313), jlai.lu (273), lemmy.wtf (164), and lemdro.id (67), whereas those exclusive to the fediverse.observer list are lemmy.dorfrollenspiel.de (79), lemmy.cafe (72), and fosscad.io (59). If the official list selectively excludes those two (as it does lemmy.world and sh.itjust.works), then the remaining difference would be 3,464, otherwise 3,674. The rest can partly be explained the sum of servers with fewer than 50 MAU, though perhaps one list is updated at a different frequency than the other to explain the rest.
For the sake of Lemmy’s growth, it’s problematic that join-lemmy.org (as an official platform) features Hexbear, Lemmygrad, and Lemmy.ml so prominently. While I dislike Reddit enough to have not tossed away Lemmy as a whole for its tankies and instead blocked those instances, featuring and recommending the use of extremist instances hurts Lemmy’s image (and Piefed and Mbin by association) in the eyes of potential users.
Platforms like Voat have disgustingly promoted fascist ideologies in the name of “free speech”, but that doesn’t excuse leading members of Lemmygrad, Hexbear, and Lemmy.ml from their persistent history of supporting Russia’s illegal invasion of Ukraine, supporting Chinese totalitarianism (not even true communism) over Taiwanese democracy, and disgusting transphobia.
I block the communities on those instances, even those I might be interested in, to instead support the growth of alternatives on mainstream instances. I still want to see comments from their users on other instances though, as many are still reasonable, and the Threadiverse is already too small as it is. I’d like to see it get bigger, and have no intention of returning to Reddit and having my data commercialized, but for as long as Lemmy is defined by those three instances, we are at a strong disadvantage in terms of user growth and retention.