Ist ein bekannter Bug, den man selbst ganz einfach lösen kann durch das Löschen von genau 2 Registry-Einträgen, die nach einem Neustart neu erstellt werden (hab leider nur den Link zur englischen Anleitung):
Opened regedit , and searched by using the search function in regedit for RmThermalProviderInfo and RmThermalProviderNum and deleted them and rebooted.
Problem solved.
Now the temps can be seen with hardwaremonitor and evga precision and rivatuner and they are all the same now like Everest.
I found this important because how would the gpu know when the fan has to run faster if the temp. readings are not accurate.
This is what i found , some explainaton :
Problem background:
Since 190.xx series NVIDIA driver's internal resource manager is trying to precache thermal sensors information in the registry during the first startup after installation. This mechanism doesn't seem to work fine yet, under some conditions sensor detection and precaching algorithm can fail and store incorrect sensor type information in the registry causing temperatures to disappear till the ForceWare re-installation.
Temporary solution:
Until the problem is not fixed by NVIDIA, there are still some temporary tricks allowing to solve it. First, you may just perform complete Foreware re-install, this will also cause precached thermal sensor information to be removed from the registry and to be re-detected by ForceWare on the next start. Second, you may manually remove corrupted thermal sensor information from the registry and this way cause the ForceWare to re-detect it on the next reboot. To do it open regedit then search and delete all instances of RmThermalProviderInfo and RmThermalProviderNum entries in the registry. Then reboot.
And let's hope that it won't take long time to fix it from NVIDIA side.