I am fairly sure both the cpu and gpu readings from software is accurate, it isn't rocket science to include an internal heat sensing probe.
Software voltage and temperature readings have never been particularly accurate, even if it isn't "rocket science".
Intel for example use a method of inferring temperatures from a chips throttling point, which will have the usual range of manufacturing variation.
Unclewebb, creator of RealTemp has written at length about it.
I've sent Unclewebb a PM asking if he can give me some links on how Nvidia and ATI do it. I hope he can help because I'm interested in what method they use.
I didn't find anything useful in a Google search. Just got tons of How Good are my Temps posts....Edited by alancsalt - 12/23/12 at 3:19pm