I couldn't quite figure that out either. It refers to the NTU Congressional Rating (most recent legislative year) which Thompson wasn't even a part of.
For me this is the most important stat:
Percent of “A” Grades
Ron Paul: 100%
Tom Tancredo: 100%
Fred Thompson: 88%
John McCain: 67%
Newt Gingrich: 57%
Sam Brownback: 50%
Chuck Hagel: 30%
Duncan Hunter: 6%
All Democrats: 0%
So, Fred's third to two VERY conservative but hopelessly unelectable guys...IOW the most fiscally conservative guy who can win!