If you have an AMD or many of Intels CPU's the core temps are incorrect. Only the singular CPU temp above the MB temp will be correct. It has to do with a value for Tjmax that is only applicable to some Intel CPUs. There are many other programs as shown above that will give you correct temps, or just ignore the individual core temps on HWMonitor.
No, its not an average, the core temp is relative to the highest temp the CPU is designed to operate and different CPU's models have different values, this value called Tjmax. It is not explained very well and I had to search quite a bit to find out what was going on. If you google "HWMonitor CPU core temp vs CPU temp" you will see the hundreds of guesses and speculations and only a few that actually explain Tjmax and its purpose ( in an AnandTech forum ). Don't worry about it, just ignore the core temps or use the other programs. I wish HWMonitor would simply turn the core temp values off if you don't have the correct Intel CPU's to stop confusing and misinforming all these people!