It is be because all feed includes Lemmy.world, Lemmy.ml and beehaw.org. That means there will be duplicate posts retrieved from other instances. If there is a Lemmy.world post they would see it 3 times: one from original Lemmy.world version, one from Lemmy.ml and one from sh.itjust.works. Bit can only comment on sh.itjust.works version because they are not logged into Lemmy.world or Lemmy.ml