Assuming the same communities are being followed and no users are blocked and all else is equal, one possible explanation is that lemmy.world is using Lemmy 0.19.3, and sh.itjust.works is on Lemmy 0.19.5. Something might have changed in how posts are sorted between the two versions.
One might expect these two instances to be pretty similar, as they both have a bunch of users and are pretty much catch-all. But in general, different instances of Lemmy will display different content by design - the users decide the direction of the instance by following communities they are interested in.
