Most OOs and private colleges accept based on region, not on particular school. Acceptance rates from magnet schools tend to be higher than from other high schools, since the AOs are more likely to have heard about these schools.
What possibly happens is that far fewer kids apply to these universities from non-magnet publics. So if a class of 700 has only 7 students applying to U Michigan, the fact that all 7 were rejected won’t be a major point of discussion among students, and more importantly, parents. However, if 120 from a class of 600 were applying, the fact that only 20 were accepted would create the impression that U Michigan did not like accepting students from that magnet high school, especially if that state’s flagship was accepting 70% of the applicants from that magnet school. Since the students and parents are all talking about this acceptance rate, the impression becomes part of the school lore.
Also, people tend to make snap calculations, and often assume that "since 80% of my kid’s friends (or my friends’ kids) are applying, that means that most of the graduating students from my kid’s school are applying. For example, most of my kid’s friends (and most of my friends’ kids who attended that high school) applied to the state flagship. I was, however, really surprised to find that only around 20%-25% of their graduating class actually applies to this university.