"contains an additional unique item" bug

From what I understand, unique items are the shiney brown items that drop that go into the Unique tab and are unique in that they are specialized and do not offer the same bonuses as other items such as rares.
I have encountered at least 2 if not 3 different strongboxes with the modifier "contains and additional unique item"
So unless this means it contains a chance shard, I feel it is broken as I have never gained an "additional unique item" from one of these strongboxes.

Confirmed this is happening in 0.2.x
I had a Strongbox with the modifiers:
"Summons Volatile Crags" and "Contains An Additional Unique Item"
I received a chance shard instead of a unique item. This feels like a bug but if it's a translation issue ("Unique-related item") that may need an update to match intent.
Bug #2,589,642,173