Nick - This is only my guess. When cars were shipped by sea, I assume some may have had extra protection sprayed on and under them if the cars were loaded on the deck where they would be exposed to the salt air and salt spray. So the cars would need to be cleaned fully to remove this "whatever". I suppose if this is true, then mine may have been loaded into one of the lower inner decks or the cargo bay, so it didn't need to be slushed because I assume that all it needed would be a waxing to make it ready for delivery and wouldn't need to be cleaned if it didn't have this extra "whatever" on the car.
But only S-T or the Heritage Center might know for sure.