Because negative and positive waves alternate in the exhaust pipe, at some lower rpm, it is no longer a negative wave that enters the combustion chamber during valve overlap. It is a positive wave, which stuffs exhaust gas back into the combustion chamber, back through the intake valves and into the intake system. This, by diluting the fresh charge that the piston is about to take in, causes a drop in engine torque. This is the “flat spot” that all engine builders and racers are familiar with. Nature giveth, but nature also taketh away.