From the Gorilla Glass website:
Power plugs are a masterpiece of glass technology and design. Glass sheets are fused together in large slabs in special ovens and then slowly cooled to room temperature. Each piece is then meticulously carved and polished using diamond wheels, steady hands, and a good eye.
Due to the way these plugs are produced, the individual members of each pair will never mirror one another, and the quality of the match will vary depending on the batch.