Actually, it isn't the water so much as the air. If flour is jostled around, it tends to compact itself, or settle, pushing out air. Therefore, there is more flour per unit volume. This isn't a problem with liquids, as they are incomprehensible. The dry, powdery incredients need to be weighed to verify that you are putting the same amount of product into the bowl each time.
To see what I mean, try this experiment. Sift flour into a bowl, then scoop out one even measuring cup, use a dry measuring cup to be accurate. Weigh it and record the weight. Then put the flour back into the bowl, with more flour, and lightly tap the bowl up and down for a half minute. Again scoop out a cup of flour and weigh it. You will find that it weighs more, as the air has been pushed out from between the flour particles. You have more flour particles per unit volume.
Moisture in the packaged flour isn't really an issue as it's in a sealed container that keeps atmospheric moisture out, until the flour is removed. Then, it takes time for that flour to absorb and appreciable amount of moisture. The atmospheric moisture can change the consistency of the crust as it's baking. And remember, the drier the air, the faster it dries out. So you need to oil, or butter the outside and cover while it's rising.
Seeeeeeya; Chief Longwind of the North