That has to do with the fact that dynomax increases the exhaust pipe diameter. Larger pipe gives less back pressure at high rpm's but negatively affects exhaust scavenging.

Back Pressure obviously makes the engine work harder to push out exhaust gases. So a larger exhaust decreases back pressure.

Scavenging acts like a suction on "non-continuous flow", like the exhaust of an engine at low rpm's. The smaller the diameter, the higher the flow velocities...resulting in better scavenging. Exhaust at higher rpm's acts more like a continuous flow, so it doesn't really benefit from scavenging.

The balance between these two principals is a give/take relationship.