I thought I would offer an analogy for another aspect of the hydration difference between sifted and unsifted flour to help illustrate the physics involved. This analogy has nothing to do with absorption like above, but is actually just a contributing factor. Imagine one cylinder, approximately 20 cm in diameter, containing a thousand glass marbles measuring 2 cm in diameter. Imagine another cylinder, of the same diameter, containing a single glass sphere measuring 20 cm in diameter. Both of these cylinders contain the same mass and volume of glass, and the glass has zero absorption ability, however, the separation of the mass inherent with the thousand marbles gives it ten times more surface area. When you pour water into the two cylinders to cover the glass spheres, and allow the water to drain from the bottom, even though the glass hasn't absorbed any water, the cylinder with the thousand marbles is going to weigh slightly more because of water tension (surface adhesion) and in some cases, like plastic (or flour), electrostatic attraction. This is just one more way a fine particulate mass is able to hold on to more water.