It's not unusual to get different temperatures between cores on a quad-core, especially between core 0/1 and core 2/3. This is because of the chips' construction, which is basically two dual-cores placed side-by-side.
Even with a perfect mount, it's still easy to get differences, as the thermal material between the cores and the underside of IHS can vary, and there's really nothing you can do about that. Personally, a 6 degree difference at load is nothing to worry about.
__________________ i7 2600K | ASUS Maximus IV GENE-Z | GTX Titan | Corsair DDR3-2133 |