I've noticed too, it makes logical sense that a plant with less water in it will take less time to dry and lose less as it has less, grapes and raisins, plainly put.
My only concern, as I'm no botanist, is if any drought stresses during harvest release or inhibit certain break downs. I recall things about live stock and stress changing the meat composition, like too much adrenalin or what not. Again I know little about this sort of thing, its easier to assume correct and the plants are well watered on the day. Probably isn't worth all the reading to find out.