Some servers choose not to connect with each other.

Those servers have users and since they choose not to connect, there will be disparity.

Let’s say I’m on server ‘a’ and you are on server ‘b’ and this post is on server ‘c’

In this example, we can say that a and b connect with c, but a and b don’t connect with each other.

We would both independently see the post on server c, and we could both leave a comment. But since a and b don’t connect, we wouldn’t be able to see each other’s comments. The people on server c would be able to see both of our comments though.

This connection is called federation and it allows individual servers to control who they interact with.

Lemmy is just a piece of software, the people running the software on their servers control who they connect with. you could for example have multiple federations that comprise tons of servers that have no direct connection at all, resulting in two totally different networks.