I don't have the answers, but I'm speculating that it has developed first from being a skeuomorphic text representation depicting a modem/ethernet port, but then just being re-used outside the original and slowly evolving to what it is currently.
I suspect it's just a stylized double-ended arrow representing bidirectional data transfer, created because Apple (or whoever) needed a localization-agnostic way to represent "Ethernet" on hardware.