Variation of resistance within conductor due to Skin effect
The skin effect is the effective reduction in the cross sectional area of a conductor that carries high frequency AC. The current density is higher on the outside of the conductor than it is an the inside. The actual resistivity of the conductor is still uniform throughout. 

Its essentially an example of back emf which is strongest in the middle of the wire produced by the changing magnetic field of the AC.



see http://en.wikipedia.org/wiki/Skin_effect