Probably a phantom reading, meaning that the sensor chip supports it but there is no sensor for it. Pretty sure that HWMonitor works the same as Speedfan in that it takes the readings directly from the sensor chip and not from the bios, which is why readings like that show up (and in Speedfan can be turned off).
Leave it open and run a game or some other system intensive program and then check the readings again. If it is a phantom reading and that 79° reading doesn't change, then you can ignore it.