Content from a community isn't pulled to a new instance until at least one user from that instance subscribes to that community. At that point, the most recent posts are pulled to the local copy of that community and new posts going forward will be, too. Older posts will not, which may be why lemmy.ca can see some piefed.social cannot.
But also, piefed supports instance-wide blocking. If you've blocked the instance from which the content comes, you won't see it from piefed. Lemmy lacks this functionality (so far).