I imagine it’s because “active” is showing recently created zines. That’s a poor algorithm for what is actually active. It should be based on number of comments and threads. It’s making it very difficult to find hip places I might want to subscribe to.
Not sure if this is the exact answer, but say you have a kbin user and a lemmy “magazine” or whatever they call them there. Once the kbin person joins the lemmy thing it automatically federated. However, it does not “back populate” the kbin. Kbin will only see new posts going forward after the federation.