Yeah, I agree, and I was going nuts trying to fix my 8C difference.
The only way I can think of for there to be a 8-9C difference in a core and it not be the chip is if there is bad contact or an air pocket between the CPU and heatsink in the area right above that particular core. In both our cases we have remounted multiple times. The spread patterns had no gaps in my reseats, and my VX is pretty dang flat now. The only other thing I could try would be lapping my CPU as well.
So, if you remounted multiple times, the only thing left, in my mind, is try lapping. But, when I lapped my VX the temps on all cores dropped my ~2C so I still had a 8C difference.