The author explains:
"The two larger parts [the ear covers] are cast zinc. Cast parts are similar to injection molded parts in that there is a tooling cost and a per-part cost. Compared to injection molding, the tool is marginally more expensive, but the per-part costs are higher, and the tools do not last as long.
"The brilliant thing here is that the two large metal parts are not mirror images of each other - they are actually the same part! This means that only one tool would need to be made to produce both parts, which saves money in tool design and number of tools. It also makes the headphones easier to assemble, since there are fewer unique parts."