I’m sure there are some good reasons, but I don’t know what those reasons are.
I’ve noticed that sometimes the instance of the community doesn’t match the instance of the user who posted there, and I was wondering why they chose to post to that community instead of an equivalent one on the instance they joined. Are there pros and cons to doing this?
That’s the way the fediverse works.
Someone posts something to a community on one instance, then it’s mirrored to all of the federated instances, and people on all of those different instances see it and, if so inclined, reply to it. It doesn’t really matter which instance it’s posted to or which instance the response comes from - it’s all shared throughout all of the federated instances.