That is a significant question.
I read about a month ago that sometimes schools toward the bottom can be just overturned based on aftershocks when schools shift at the top; because the number 1 school *must* have an overall score of 100 and the very last school has to have a 0? (or something like that... the whole rankings formula is kind of nutty)
Anyway, when a school drops 17 in two years, that is worrisome. For that matter, take even UNC, this school has dropped 11 spots in 2 years. It dropped 9 spots last year and another 2 spots this year, going from 25-36-38. And there are apparently some serious problems going on there, because they can't even get their act together when it comes to admissions. (look at LSN and you'll see what I mean, I think they maybe admitted 30% of their class and it's going to be April)
There is a methodical measure to these schools, as simple as if the schools below Miami suddenly got overall scores higher due to increased LSAT/GPA or peer evaluation scores, or whatever the heck went up, that's invariably going to bump down Miami if they didn't do anything to go up.
It'd be more worthwhile if you could get a hold of the total numbers for US News's evaluation of Miami for rankings from 2007-2009 and see what's changed. That's going to be more indicative that the school's overall ranking.