It's mostly based on when the first PieFed user joins. But when they join PieFed attempts to mirror the last 50 posts in that community and if it's a quiet community some of those posts will be very old.

Pulling in those 50 posts works most of the time but sometimes, for unknown reasons, it fails and only some posts get mirrored.

@rimu@piefed.social might be able to answer better and more specifically since he is the admin for piefed.social, but here is some info on what might be happening.

Federation in piefed works the same as it does in lemmy...in that piefed won't get posts for a community unless a local user is subscribed to it (lemmy provides the most recent 20 posts to an instance when the first subscriber joins). So, until a user from piefed.social subscribed to those communities, then no posts would show up. As a corollary to this, if there is one local subscriber that then unsubscribes, posts will stop federating over. So, that can cause things like gaps in federation.

As for automatic deletion of older posts...this is something that a piefed admin can do on a community by community basis. When it is turned on for a community, that retention policy is listed in the community sidebar. For instance, if you look at the sidebar of !anime@ani.social, you will see that posts are retained on piefed.social for 365 days.

The communities you have listed don't have a retention policy in the sidebar, so I don't believe this is the issue. Most likely it is due to federation not starting until a certain point.

I know that rimu was actively working on an archiving feature for piefed, but I think this feature isn't live yet. The archiving would basically "archive" old posts in object storage instead of deleting them. So, if somebody does try to load up an old post, it might take a hair longer to load as it is unarchived, but it wouldn't be deleted and inaccessible.