The FAQ from SpeedFan website tell you basically this:
How can I identify my CPU temperature?
To find your CPU's temperature sensor you can leave your system idle for a few minutes, to let temperatures drop, and then go to 100% usage for a while. The temperature that rises faster is the one you're searching for. Other available temperature readings usually come from your sensor chip itself, from the southbridge, the voltage regulator, or even from an additional probe placed under the processor. This additional temperature sensor is not necessarily a duplicate. Some CPUs are not actually able to report the internal temperature from their die. To be able to read their temperatures, an additional external sensor (thermocouple) is used. In such cases, you will see two temperatures referring to the processor. The higher of the two is from the die. As a final note, please remember that not all available temperature sensors are actually connected to something. If you happen to read unusually high or low temps, they are likely to be from a disconnected (unused) temperature sensor.