Air can only absorb a finite amount of moisture before it is saturated. At that point it condenses on cool surfaces. As the air goes through your box and picks up moisture from the food it gets closer to that point. Imagine the air as a sponge (kindof). As you slowly wet the sponge it will absorb water very fast at first. After a while it will slow down as it gets wetter, or more saturated. Just like air does. So as the air moves through the box it will always dry the food faster at the first trays it moves across.
