Votes can be weird as hell with federating. If that's what you're using to gauge activity, don't be surprised if they don't federate at all, or take unusually long times to show.

As far as instances of choice, there's no hard and fast reason to either separate or contain things like writing. Unless you use a pen name you want kept apart from your other stuff. It likely does make it easier to keep track of things you write, since if you comment a good bit, it can be harder to find previous content you made among the casual conversations.

But it is a good thing to keep at least two accounts on different instances in general. Redundancy is always helpful what with instances going down or closing entirely. So, if you do that, there's little or no reason to not also keep some kinds of content separate as well.

Federation is when two servers agree to share information using the same protocol. The best example of this is email. Different providers can send emails in the same format to different servers, regardless of whether they have the same owner because they use the same protocol. This is federation. Federation also has the plus of being able to choose which servers you want to share information with.

You're probably right that one instance isn't federated, or another option is that one server has an older record of a post than another and hasn't been updated yet for some reason.