The midpipe section of the exhaust system has a crossover where the exhaust from one cylinder assists the exhaust flow from the other cylinder. The design of an exhaust system is a complicated subject, but in short, the crossover section size, placement and even existance depends on the choices made by the exhaust system designer regarding shaping the torque and horsepower curves, as well as the result of exhaustive (pun) dyno work by the manufacturer.