For fine-grained soils there will be a microscopic water layer bound around the grains due to some electrochemical process. This water is called adsorbed water or hygroscopic water. Adsorbed water fills the voids of the soil. For clayey soil, the adsorbed water is in the diffused double-layer form.
The presence of adsorbed water around soil grains consumes a considerable amount of void space. It will intact reduce the permeability of soil though already voids contain some amount of water. Due to the reduction in effective pore space, water is not free to move through soil and thus reduces the permeability.