Ancient coins from skelly ship loot???

As the title suggests, I somehow got 25 ancient coins while selling loot from a skelly sloop I fought. I had set sail for lost shipments but couldn't find the starting clue when a sloop spawned on me! Having nothing to do, I quickly sank it and sold the treasure, but as I was selling I saw, among the gold notifications, one that said 25 ancient coins. Not sure why or how, but if anyone knows what happened can you please tell me?