I7 920 - One core fairly cooler than others

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

You're right at that annoying temperature mark, too. Nothing beyond 70 makes people comfortable except maybe for modern GPUs.

I mean, you can try reapplying and re-seating the heatsink but I don't see how you could effectively cover 3 cores with TIM and somehow miss one core... so I just want to blame it on heatsink surface irregularities.

Oh whoops. Yeah, you're right, I kept thinking it was the other way around even though I read it right the first time.

What I would give to be able to see the imprint your TIM left behind... then we could actually tell what cores were covered or not, and whether the issue was the TIM or... what.