The website does chances based completely on numbers that they get from a few different places. First, is the information that schools report and the second is the from people who get into use cappex/report their acceptances to cappex. That is how they calculate your chances.
And honestly, it would be accurate if the college only looks at their students empirical data, but we all know that that isn't always the case. And also, the fact that they at least partially rely on data reported 18 year-olds is also mildly disconcerting.
But as long as you take the results with a grain of salt, then they can paint a good picture of your chances.