Why is Cornell sometimes ranked below Northwestern and Vanderbilt? Is it base on graduate incomes or the quality of professors?
Assuming you are talking about US News rankings. It isn’t one factor. They have a whole methodology for their rankings. Here is more info on it:
http://www.usnews.com/education/best-colleges/articles/how-us-news-calculated-the-rankings
The rankings between these 3 schools are irrelevant in my opinion. They are fairly close in quality of education.