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.
Jewelry is measured through a gauge system (see chart below). The higher the gauge number the smaller the wire is. A standard "earring" is usually 20 gauge. Jewelry gauges/sizes can vary between manufacturers. There is no universal regulation on companies to make a standard size in body jewelry. A big part of this is that some jewelry is made overseas and measured in millimeters. And in the USA jewelry is measured in inches/gauges. So many times jewelry can be anywhere from .5mm to 1mm different.