I think in some fields you can't really have a safety school because there are so few schools in your teeny tiny little specialty. I know, for what I want to do there are maybe the schools in the US and maybe another 10-15 globally. While they're not all tied by rank, the rank becomes meaningless because it is such a tight knit community and each school is funded enough to support what can be a costly type of lab.
Instead, each school may have slightly different parameters or weighting standards when eyeing up potential candidates. I think in a small field like this, networking, personal contact, and the random element (so-and-so wants to diversify in this way, so-and-so is looking for this specific lab skill) trumps rank in determining which school will accept you.
For this reason, when I decided to add a "safety school" or what I've come to think of as a "wild card" school - I had to choose one where I would do something entirely different, more likely for a Masters and not a PhD, with the idea that if the universe decided I shouldn't do Thing A - I would do Thing B and see if a few years later I missed Thing A at which point I'd give it another shot.
Edited to add: incidentally my safety school is the only school I have yet to hear from, I'm guessing they could sense that I had to force the fit in my SOP