Everyone is forgetting that the integrated heat spreader is *soldered* to the core. The IHS is NOT the core!!! And it's not just "touching" the core either, othewise the heat transfer would be pretty bad.
In the old days (Pentium 4), the IHS was simply attached to the core with thermal material mating the two surfaces, and all you needed to do was take a razer blade and pry with a certain method at each corner, and proper technique (required study) would pop the IHS right off.
Starting with Core 2's, Intel started soldering the IHS to the core, meaning popping it off would rip off part of the core with it (some people found a way to melt the solder very carefully, but even THAT was risky, and rarely worked).
The problem is that the solder-IHS surface isn't always perfect, so you wind up getting variations in the core temps. Lapping can improve temps a few C, but will rarely make the cores all level.
For comparison, my third core is my coolest core (at idle) while my 4th core is the hottest (9C hotter than the third core), and the first core is second hottest. But at full load, the first core is the *coolest*, with the second, third and 4th cores being almost the same.
Have a look for yourself and see:
Here's it at idle so you can see the difference (lower speed tho: my 24/7 settings)
Edited by Falkentyne - 4/10/11 at 2:07am