Bigbmxdave: Please don't post useless nonsense like that.
I assume it's because they both use different methods of measuring. I first thought that all programs should use the same basic motherboard function to measure the temperature, but I have also experienced differences between software.
Are you using both Everest and Speedfan at the same time, because if not, it may be because of a difference in the load on your CPU. Just a thought.