The main system has been designed to provide more than enough airflow for the entire house. So when there is weak or insufficient air flow in one area there must be excess flow in other vents. Once the booster fan is installed in a problem vent that doesn't get enough flow, it will reduce flow in other areas especially in areas with excess flow. But since the flow change is affecting the entire ducting system, it won't be noticeable and the temperature difference in those areas with excess or enough flow is not even measurable by regular temperature sensors.