Brown University costs $73,892 a year—but here’s how much students actually pay

Perhaps this is a silly question, but, how do they know? Clearly, if a student applies for financial aid the school would have parental income. But for full pay parents who do not file for FA - how would the school know this information? In this case, for the 19% at Brown that they identify as the highest 1% of American earners, it seems unlikely they would have “bothered” (for lack of a better word) filing for FA. Thanks.