Answer
Air is compressible, unlike water. Boyle's law tells us its density is proportional to its pressure (at a fixed temperature).
Down low, where the pressure is greater, air density is relatively high. In our model this corresponds to foam bricks that are squashed a lot and whose density is relatively high.
At high altitude, where the pressure is less, the air’s density is less. In our model this corresponds to foam bricks that are not as squashed.