It could be just for their track setup. I don't know the advantage or disadvantage of either setup, but I'd imagine single pass would be cooler.

Harrop kits are back :)

Assuming pumping flows and everything else are the same (even going into the fins per inch within the HX itself) it would result in less cooling. As well you'd have a temperature gradient across the core itself as water flows from front to rear (or vice versa).

Now with that, they could have been able to increase pumping flow across a single pass to up the capacity of the heat exchanger, or add more fins per inch (at a negative impact to airflow) to increase heat exchange effectiveness.

Or, it can be something crazy like a 3-pass system.