Core temperature, dual core, only one core temp changes

I recently upgraded my heatsink and before hand I only either check the temp through bios or through an older program that only showed on temperature. I not have tried out 4 different programs, all of which show the temp for each core independently. My processor is an E8400 and core #0 temp changes with load, had it as low as 26c to as high as 50c. core # 1 is always at 36c. Therefore the average temp which is, shown in bios, is always artificially high under idle. What would cause this?8