ok ,i think i can let it simple,, if you take a cross section in the wire, and plot its magnetic flux density(using maxwell equation) , you will find that it is concentrated in the middle of the wire cross section,and smaller density at the surface,so, inductance in the bulk of the wire(in its middle)will be greater than at surface ,due to coupling of more flux,and as resistance proportional to inductance(XL=ω*L),so at high frequency ,this effect begins to appear,resulting in higher resistance at the middle rather than at surfaces.......also you can prove it using maxwell equations...