Yup just like poster above me says, it is because the actuator valve on the driver side muffler remains closed under 3000rpm if part throttle. You can notice this when you do a cold start on the car - the exhaust condensation will clearly be seen (or felt if you put your hand behind the exhaust tip) coming from only the passenger side.
There are various ways to disconnect/block the vacuum line to the actuator valve so that the exhaust flows through both mufflers ALL the time. See the DIY section for some ideas. I used a pin valve in the vacuum line so that when I close the pin valve, the actuator stays open all the time and I have both mufflers working even under 3000rpm. It is a much nicer sound driving around town... throatier and more rumbly but still smooth as is characteristic of the I6. But on long highway trips where you are on and off the throttle there is drone and so the pin valve works great - for such a long trip you loosen the pin valve and in so doing you are back to stock so that the actuator valve opens/closes as programmed. Best of both worlds.
