Maybe they "overbuilt" them, for the draw weight? In Arizona, humidity can go from 10% to 80% plus during monsoon season. I had a bow go from 44 pounds draw weight to upper 30's, 38 if I recall, in one hunt.
I sort of dry the bow out, high over a campfire coals in the evening, during the really humid days of our summer velvet season (also like how the smoke as I had some damp twigs adds patina to the whitewood).
As for adding moisture, I'm with you, perplexed.