It's my understanding that many of the factors that schools are ranked on come down to the school's reputation, and that primarily PT Department Chairs are the ones who fill out the rankings questionaire. So, typically, if a school's faculty are well respected, and in particular, are well published, a school tends to be ranked highly. Shirley Sahrmann, PT, PhD, FAPTA, for example, is widely published, and in addition to contributing to the body of PT scientific literature, she has also started a Movement Science post-graduate Fellowship. She, along with other prominent faculty members, is quite probably why Wash U is ranked highly. So, schools with faculty members that are less well published, don't hold PhDs, etc. tend to be ranked lower.
In defense of my alma mater, Arcadia University, it is ranked highly (and "deserves" its ranking as much as any of the other highly ranked programs) quite likely due to a faculty that has been incredibly well-published and stresses evidence-based practice and integration of current best evidence into treatment. Some of the other places you may visit may have shiny, more expensive equipment and a larger campus, but most of that is window dressing. To borrow a phrase from Jason Silvernail, a PT that posts regularly on evidenceinmotion.com and somasimple.com, "The only high-tech equipment I've ever needed in the clinic are my brain and my two hands." A nice quote that I couldn't agree more with.
My $0.02.