In this case:
The applicant was rejected by M&T but accepted by SEAS (CS) and Princeton, and decided to attend P. Theoretically, P wins Penn in this cross-admit but who knows if M&T would win over P. Many applicants for Penn are for Coordinated Dual Degree programs. If they don’t get in CDD but in a backup, then, most of them may go to other options. If considering very low acceptance rate of these CDDs, hundreds, if not thousands, admits may give up their backups and go to other options, most would be HYPSM.