I've got another crazy theory: That the cars were driven on to the ships in more-or-less the order they were built--so the first built, first on. But, they come off the other way (yes, it's a lifo, or a stack). Makes some sense if the Ship-With-600 has docked yet (which I didn't think it had)...or if a ship that left later showed up sooner.
So, the latest guys to order are getting their cars processed on the dock first, and us earlier order-ers will be processed at the end of the queue (see what I did there, I went from a stack to a queue).
Grasping at straws, searching for hope. Just as early man came up with mythologies to explain their baffling world, so are we here.