I suspect it's because they are on different LAN addresses, so the NIC just does it's usual thing and transfers the traffic onto the LAN and then receives it back again. I don't think this would happen if there were two processes talking via the loopback address inside the NIC driver.
PS Actually, I know that to be true, as I run the SMP Folding@Home client on my Mac, and that runs four concurrent threads, communicating with each other via the loopback adapter. They generate about 100 MBytes per sec of network traffic, none of which is visible on my router's traffic lights, and which has no discernible impact on my Mac's real network traffic.
Last edited: Jun 25, 2007