It means you weren't the first person on your server to subscribe that community/magazine.
Prouvaire
It would be nice to see people engaging with old posts when they stumble across a community and subscribe to it.
One barrier that will make this difficult is that instances only get a community's feed from the moment they first subscribe to it, if that community's home instance is on another server. So if you're a user on - say - leminal.space and you're the first person on that server to subscribe to - say - Musicals@kbin.social then you will not see any of that community's old posts, only posts created (or boosted) after you've subscribed. This makes it difficult to engage with old content unless other people on your instance have been members of that community for much longer.
This is one of the issues with the fediverse model that doesn't exist in a centralised model like reddit. And - sadly - smaller, niche communities are the ones most likely to be affected by this limitation, because they're the ones least likely to be federated to a large number of instances. It makes smaller, less active communities look even more inactive than they actually are.
You're not the only one who's picked up on this:
https://www.primetimer.com/quickhits/the-west-wings-hartsfields-landing-has-never-made-any-damn-sense
If you watch the show (and I've watched it a lot) you can tell that Aaron Sorkin wasn't writing to a detailed plan. I think a lot of his ongoing storylines grew organically (and were often turned in very late, which ultimately led to him being sacked from the show), which meant he sometimes repeated, or retconned, beats, or made other mistakes.
Hartsfield's Landing was probably named after Hart's Location, another New Hampshire town that has often votes at midnight. I recall reading a story that, because of the weather, of the three towns that usually do vote at midnight only Dixville did so this year.
edit: Hrrm. Turns out that's what the linked CNN story actually says. Serves me right for not RTFA.