They're wired backwards so that they invert the signal (allegedly... this seems like the kind of thing that might vary depending on who's on the workshift at the factory that night). Ie, a sound that starts with an increase in sound pressure at the mic starts with a pressure vacuum at the eardrum, assuming phase was preserved through the recording. It's theoretically audible but as long as its inverted the same way in both ears, really not a significant issue with real-world audio content.