I think that the international student aspect is a huge factor. Emory recently made the decision to cut their admissions of international students from 40%+ of the total class to around 30%, and their employment statistics subsequently shot up to 97%.
It's also important to look at the specialties of each school. For example, Northwestern is great for rotational leadership programs, but the availability those positions is not as high as it used to be.
Regional changes could also be an issue (Detroit's bankruptcy) but I don't think that it's too much of a factor...
IMO the international students is the highest.
I'm curious - has anyone ever correlated the top business school's percentage of international students with percentage employed 3 months after graduation?