Air behaves like a liquid.
Imagine a glass with water in it. At the bottom are granuals of sand. The water and sand has a weight. Stir the glass up (supply energy) and the sand becomes suspended in the water.
The sand and water still weight the same. This is despite the heavier granules of sand not touching the bottom.
Essentially the geese are disolved in air after they have supplied there own energy to move around the system.
This is the same as helium balloons and lead bars being essentially dissolved in the same air.
At the surface of Earth air pressure is the result of all the particles above in a unit column of air. If these particles increase in mass or weight then pressure at the surface increases. It's the same inside the aircraft.
Mariner9, I take your point about acceleration being able to change.