@calmom - with regard to the difference between the two Peace Corps ranks:
The direct report from the Peace Corps divides schools into three ranges based on undergraduate enrollment. It then ranks schools within a range based on the absolute number of Peace Corps participants. : The ranges are: under 5K, 5-15K, and over 15K
This methodology favors the schools at the top end of the enrollment range relative to those at the bottom end of of the range. Since there is a factor of three difference in size between the top and bottom of the middle range, the smaller schools may not appear on the list despite having a higher percentage of graduates in the Peace Corps.
The list in your link only covers the top 5 schools in each range, which tends to amplify the bias. For context here are the undergrad populations for some schools.
Cornell 14,500
GWU 11,000
American 8,000
Georgetown 7,500
Tulane 6,500
William and Mary 6,300
Tufts 5,200
Historically, the Peace Corps has published a larger list and all of the schools I have listed are on it. Occasionally, Tufts will not appear due to its small size. It is unusual for Cornell not to appear given its large size, so I suspect that Cornell may have grown and is now in the “over 15K” range. With respect to your observation on Georgetown and Tufts, they are both noteworthy for the strength of their undergrad IR programs (as are GWU and American) so I believe there is a correlation between IR and Peace Corp participation. This would make sense given that IR grads tend to have a more global focus and IR tends to be a feeder into careers in NGO’s.
The Washington Monthly methodology divides the number of Peace Corps participants by “total enrollment” (but they do not specify if “total enrollment” includes graduate students), so the smaller schools within a range tend to move up and the larger schools within a range tend to move down relative to the Peace Corp list.
Note that the Washington Monthly data is usually a year behind the latest direct report from the Peace Corps and the number of Peace Corps participants per school varies from year to year. If one wanted to be more rigorous one would take an average over a couple of years.
For reference, here is the Washington Monthly methodology
https://washingtonmonthly.com/magazine/septemberoctober-2016/a-note-on-methodology-4-year-colleges-and-universities-7/
and Tufts 2012-2017 rank in the direct reports from the Peace Corps
http://now.tufts.edu/articles/head-volunteering
http://now.tufts.edu/articles/tufts-ranks-high-peace-corps-volunteers
http://now.tufts.edu/articles/tufts-university-ranks-high-peace-corps-volunteers
http://now.tufts.edu/articles/tufts-grads-active-peace-corps
http://medford.wickedlocal.com/news/20170305/peace-corps-recognizes-medfords-tufts-university