Enrollment data from http://en.wikipedia.org/wiki/Ivy_League#Members
The Total as % column represents the percentage of all Ivy League students that are enrolled that that particular institution, and the Rescaled ngram frequency column represents the percentage of all NYT Ivy League wedding announcement mentions since 2010.
For example, 19.2% of all Ivy League students are enrolled at Columbia, and 23.3% of the NYT Ivy mentions were of Columbia, so the difference is 4.1%
It’s worth noting that Brown is also a fairly common surname, and there’s no easy way to separate mentions of the name from mentions of the University, so Brown’s true rescaled ngram frequency is probably be a bit lower…