Why the permeability and porosity of the silty sand is so different from the permeability and porosity of the well-sorted sand. ?

this is due to difference in grain size , in well sorted sand stone all the grains are off same size so excellent porosity and permeability but in silty sandstone there are silt size grains that fills the pore spaces between sand grains which decrease the porosity 

the main reason is because of silt particals...silt reduce the permeability.porasity is is also decreased.

because the well sorted sand higher in porosity  its packing is cubic shape and may have an effective porosity but in silty sand the silt fill the pore spaces so have lower porosity