The temps can be hotter or COOLER by 5-10C degrees. Gigagbyte, or maybe it was Tyan, messed up a temperature correction algorithm on one of their bioses and was way off once. I also see many posts in here about people getting different readings from Sisoft, Sandra, the Bios, the MB manufacturers monitor program, etc. The accuracy of the temp reading is no better than the software "digitizing and correcting" the analog signal. I've seen the same CPU/HSF in the same case, with the same case and outside temperatures read from 45-55C when switching between 3 different MBs.
The next big contributor is whether the bios uses an external thermal sensor under the CPU or the thermal diode on the CPU. That varies by MB, CPU type, etc. The "under the CPU" sensors can get dirt and finger oil on them, be pushed down during CPU installation so they are not actually in contact with the CPU, etc.
The third factor is the accuracy of the (external or internal) thermal sensor. These parts vary a few percent from lot to lot when built. The accuracy of the thermal sensor will also change as the thermal sensors ages, gets heated and cooled and undergoes vibration (from the fan). Typical commercial parts rated at 2% initial accuracy will go to about 12% at "end of life".
Bottom line, be conservative. The thermal readings you see are not that accurate (even more so with AMD CPUs than Intel). Aim for a system temp of ~35 and a CPU temp of 45-50. Then if the sensor is reading low 5-10 degrees you are safe.
The one thing the sensors are reliable for is showing you a relative change (like if the system is getting dusty or a case fan died.)