I should have also said that all cores are running with approximately
the same "load", as reported by Core Temp.
A different temperature monitoring program reported the same thing.
Conjecture only!
The heat sink to the chip compound is not uniform
The heat sink or chip surfaces are not flat enough
The heat sink does not have a uniform size, or the air flow is not uniform.
There also are some things that have to do with how the chip is mounted
inside the case, and what is adjacent, such as circuitry that gives off
more heat.
Finally there is the possibility that the sensing devices inside the
chip are not uniform in accuracy, or the supporting circuitry on the MBD
is not quite what it should be in an ideal world.