The REAL reason: http://www.techpowerup.com/realtemp/docs.php
No seriously though, everyone should read this. I was worried / confused too until I found this. It's not that one core is working harder, or not as hard, as the others.... its just an inaccuracy/precision problem with the DTS.
Some important highlights:
Originally Posted by RealTemp
The first problem is that TJMax is not a clearly defined value. Intel released a lot of information last year but they now refer to this data as TJ Target. Actual TJMax for any processor can either be equal to TJ Target or in some cases it might be slightly higher than the Intel listed TJ Target. Unfortunately, Intel did not provide the user community with any information about how much TJMax tends to vary. Based on my testing and reports from users, a 5C difference doesn't seem too unusual with 65nm and TJMax might vary by 10C in the 45 nm CPUs. The result is that two CPUs with the exact same model number might have different TJMax values and it also seems that there are situations where cores on the same CPU can have different TJMax values. For the 45nm processors, TJ Target and TJ Max seem to be similar but for the older 65nm CPUs, actual TJ Max can be 10C or higher for many processors.
The next issue that effects all of these sensors is slope error. That is when the data coming from these sensors moves at a different rate than the core temperature is changing at. The Calibration feature in RealTemp is used to compensate for sensors that move along different temperature curves and once again the Core i7 sensors seem to be greatly improved with only very minimal slope error in the normal temperature range.
Hope that helps!Edited by donkrx - 8/12/11 at 5:03pm