According to data posted by Greg Mankiw, it may be because I went to Swarthmore College, the undergraduate institution that produced the highest number of economics Ph.D's per capita. Clicking through to the study by John J. Siegfried and Wendy A. Stock, I see that this covers Ph.D's earned between 1966 and 2003. For much of this period, the late Bernie Saffran was an economics professor at Swarthmore. It will be interesting to see if there is a drop-off in Swarthmore's ranking in later periods.