Every night the body gives off moisture. That is normal. A good duvet handles that moisture without you noticing. That is how it should be.
What down does
When down meets moisture, it absorbs enough to catch the night. So it feels less quickly clammy than materials that leave moisture where it arises.
What happens by day
By day, when the duvet lies open or is carefully aired, the down releases that moisture again. The clusters recover their volume. The lightness returns. Ready for the next night.
What synthetic cannot
Synthetic fillings take up moisture far less naturally. The moisture builds up faster, so the bed can feel clammy and the material recovers less well.
And the fill power
A lighter filling with good fill power can be warm without becoming heavy. It leaves more room for air circulation, and that air also helps in handling moisture.
Why freshness is so quiet
A bed feels fresh when moisture gets no chance to stay. You do not notice that as a striking effect, but as the absence of a clammy feel. Down does its work best when it may dry again during the day.
The role of air
Airing is no ritual for show. It gives the filling the chance to release moisture and regain volume. So the next night does not start with the remains of the previous one.