Having spent several years hunting smelly humans with SAR dogs, I know the reason why humidity is important for scenting conditions.
What humans call "scent" is aerosols of tiny particles PLUS gases produced by bacterial action on those particles, on your body surface and on your airway all the way down to your lungs.
This bacterial action is highly important, and it explains why humid days produce better scenting conditions. The stink-producing bacteria like to be somewhat moist and somewhat warm. (Think of Goldilocks sampling the three porridges.) When it's too dry, they shut down. When it's too hot or too cold, they shut down. When it's "just right", you stink up the woods.
Particles of dead skin are constantly sloughing off your body, both from your skin and from your airway to your lungs. They fall to the ground at varying distances depending on the wind. Some fall where you walk. Some are blown onto nearby vegetation. Some get lifted over hills and spook critters downwind that you will never see as a result. Some get blown out of your treestand and over the heads of deer that walk under you. (This is a major reason why treestands are successful. It has precious little to do with your scent control, IMO.)
Why do I say precious little? 'Cuz if you're breathing, you're constantly exhaling stinky gases and particles from your airway on which the bacteria can feed, thereby producing scent that an animal can detect and identify.
(And if you're not breathing, a SAR dog (or deer or bear or pig) can still find you. Your body is decomposing from bacterial action. You continue to stink, but with a different odor now. That's how dogs find drowning victims, dead avalanche victims, and homicide victims buried in the ground.)