Actually, from my understanding, that statistic comes from Fisher, B.S., Cullen, F.T., & Turner, M.G. (2000). The Sexual Victimization of College Women. National Institute of Justice, Bureau of Justice Statistics... (linked to google page, PDF in first link)

Page 10 (17 of the document):

At first glance, one might conclude that the risk of rape victimization for college women is not high; “only” about 1 in 36 college women (2.8 percent) experience a completed rape or attempted rape in an academic year. Such a conclusion, however, misses critical, and potentially disquieting, implications. The figures measure victimization for slightly more than half a year (6.91 months). Projecting results beyond this reference period is problematic for a number of reasons, such as assuming that the risk of victimization is the same during summer months and remains stable over a person’s time in college. However, if the 2.8 percent victimization figure is calculated for a 1-year period, the data suggest that nearly 5 percent (4.9 percent) of college women are victimized in any given calendar year. Over the course of a college career—which now lasts an average of 5 years—the percentage of completed or attempted rape victimization among women in higher educational institutions might climb to between one-fifth and one-quarter.

To me, this seems like dubious statistics, but even if you just use the 6.91 months for 4 years (a normal college experience from what I've seen), you still end up with 11.2% of women being victims of either rape or attempted rape in their college careers.