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Well, on Reddit people often do create new communities for the same topic because they don't like the rules/culture/mods of the original one. So would you say that's the same reason for choosing a different community on the Fediverse?
Does that affect interaction? Perhaps this is a difference between platforms? When I signed up to PieFed, I chose some interests and it automatically subscribed me to various communities, some of which had the same name but different instances (for example five different communities named "Games"). I don't know much about how community discovery works on any of these platforms to be honest.