When all of the items are removed from a shulker box, the item NBT tag remains despite it being unnecessary.
- Get a shulker box that you've never used before. You will notice it has no NBT tag.
- Place the shulker box.
- Place an item in the shulker box.
- Remove the item from the shulker box.
- Break the shulker box. You will see that it has 1 NBT tag that it didn't before any items were placed in it.