I think it's the opposite. The people who accessed the left hemisphere area he mentioned were simply recalling the previously learned fact that 7-3=4, while those who accessed the right brain area that processes matters of quantity were the ones who were actually trying to figure out the difference between the two quantities, 7 and 3 from scratch. The latter is much more time and energy consuming.
It sounds to me like the moral is: the people who do better on the PSAT would be those who don't have to re-invent the wheel each time a wheel is called for. They just pull one off the shelf.