I read an instance will only mirror content after an user requests it, so different people may have different requests.
for example if I want to follow a community about gardening on lemmy world, and nobody else from my instance is following content from lemmy world, then my instance doesn’t show lemmy world- until I subscribe to a community. Once I do, my instance will start receiving lemmy world content, and everyone else from my instance will be able to see lemmy world (even if they dont follow anything there) if they browse All.