I checked my temps in the morning when the temps in my case and my processor were most likely at the ambient of my room. It turns out that at idle the mobo is misreporting my case temperature by approximately 3-4C (nxp101 says 27C while my mobo reports 31-32C). This is at ambient room temp of about 24C taken at the other end of the room.
So it seems at idle my mobo is misreporting by a lower number than at load. This seems extremely unlikely and wrong.
I'm sure Russ will respond if he's not back climbing a mountain somewhere, but I don't see why this is extremely unlikely and wrong. Nonlinearities in thermal sensing devices are pretty common. Also, unless the 2 sensors you mentioned are in exactly the same position, they will likely be subject to different secondary heat influences (on the board, from peripherals, etc) at different loads.