I generally blame the sensors or the chipset coding interpreting the data from the sensors before blaming the hardware monitoring software. The sensors are read by the motherboard/chipset and the software simply gets that information from there.
The mere fact both Speccy and HwMonitor are showing the same 9V confirms that it is not a software issue.
What does it say the voltage is in the BIOS Setup Menu?
I would not call the sensors "afterthoughts", but they certainly are low-tech and cheap - that's cheap as in inexpensive, as opposed to poor quality or likely to fail or fall apart prematurely.
Your power supplies are not putting out 9V or your motherboard just would not function. The ATX Form Factor standard allows for just ±5% tolerance on the +12VDC rail meaning the +12V must not drop below 11.4VDC or go above 12.6V. Motherboard regulator circuits can compensate somewhat, but I doubt they could for a 25% out-of-tolerance condition. Certainly not without some extreme stress and thus excessive heat situations.
And hard drive motors would have a very difficult time spinning up too - if able to at all.
So your PSUs are putting out close to 12V or else your computer would not be booting up (and you confirmed this by trying 2 different supplies).
I suspect this is due to a simple coding error in your BIOS firmware used to calculate/interpret the readings from the thermal diode sensor. The sensor could be bad, but typically a bad sensor gives no reading, or the reading jumps all over the place and is not consistently wrong by the same amount.
I recommend you check your motherboard's website for a BIOS update. If none, and your temps are all okay, I would not worry about it.
Oh, and just FTR, that's bus
, not buss.