I use Speedfan myself, but the 20 degree difference is likely due to one program using the socket sensor and the other using the one that's built into the cpu.

Valve adds CPU temperature monitoring to Steam's Performance Monitor for gamers to track potential issues. The feature is included in the latest client update with minor patches and is available for ...

In a nutshell: The September 9 Steam client update is one of the largest in recent memory. It introduces dozens of bug fixes and several new features meant to improve the user interface, controller ...