The speed of evaporation depends much more on a velocity of air flow. If I direct an air outlet to my head, my skin and my eyes could be dried several times more (e.g. 300%) whilst decrease of evaporation between old and new a/c (5 vs. 15%RH) is about 5%.
There can also exist some kind of psychosomatic effects in 787. If you feel calm and relaxing (thanks to advertising) your breathing is light and your respiratory system is dried only little. When you are feeling uncomforted and upset, or you worry about it, both heart action and breathing is much higher and your respiratory system is dried more.