Was your system idling, when you read the below-room-temperature reading?
If so, that's, uh, actually a good thing.
There are two types of thermal sensors: Socket and Core. Socket temperatures are great for when idling, but tend to read 5-15C higher when the CPU is at load. The CPU Core temperatures are modified by a type of equation to offset this, so they are actually the most accurate when the CPU is at load, but tend to be strange (often below-ambient) when the CPU is near-idling.
I have a Biostar A85W motherboard housing an A10-5800K, and I get no less than four (4) different temperature readings. One is the CPU socket, another is the CPU Socket (which reads through CoreTemp), another is based off of the CPU Core but reads +10C higher, and, last but not least, another has crazy temperatures that range from 60-90C.
AMD Overdrive should read CPU temperature - you may have to select the "CPU Status" tab under Status Monitor. As a warning, AMD Overdrive now measures thermal margins, rather than giving a straight reading - so it given you a reading based on how close you are to a certain limit.
Anyway, put your CPU under a load (preferably with Prime95). Depending on your case, airflow, etcetera, I wouldn't be surprised if the reading was somewhere between 40-50C with CoreTemp.