Could be an irregularity in the surfaces of the heatsink and the CPU. Several heatsinks have surfaces that slope upwards or downwards or one direction or the other. And yes, it could be the way you applied your thermal compound, but unless that 7 degrees is making you reach dangerous temperatures, then I wouldn't bother with it.
One of my cores remain consistently 2-5 degrees higher as well, while the another one is about 1-2 degrees higher than the other 2, which remain identical in temperatures. That one core is preventing me from hitting a 4.0ghz overclock... T_T