Here is a good anaolgy of what is happening to the air inside the top without the diffusers. Go flush your toilet and watch the water go down. See all the swirling going on and the empty area in the middle? That empty space has no volume or density to it. Now, take a top that has diffusers in it to straighten the air and you end up with a higher charge in the same size cavity. That is a real layman explanation
Paul