It is a feature of how low variable numbers (the BP / Wrists) combine with large static numbers (The augments in the type 9 slot.) The wrists have disproportionately high AC and it all balances out. If you swap from 8 pieces of Castaway to 8 pieces of Crypt-Hunters (And aug appropriately), you will gain AC. If you compare individual pieces you will experience what you see. If you swap wrists, arms, and other low AC pieces, and keep the Castaway in high AC slots (Chest/Legs), then you will have the greatest possible AC (Of those sets).
