Huh, you woulda thunk that USNWR would send an Achievement Award to all the recognized schools?!
Of course, many colleges are now following my alma's lead and NOT participating in the inane college rankings surveys performed by USNWR.
The methodoloy is flawed, based soley on AP test taking. I couldn't take AP chemistry, but outscored my classmates who took AP chemistry on the Chemistry SAT subject test. The next year, I could take AP Physics, and got a "5", the highest score.
I was taking AP-level German (I won the German award for the Phila public school district) but didn't bother with the AP test.
These tests cost $50 back then, and I can't recall whether we got any assistance or not, but imagine today, if you took 4 AP classes, at $100/pop, if you are a disadvantaged student, you may only take one. These tests are also fairly grueling, NOT mutiple choice, as was the old SAT.
My education, however, was the same, whether I took the test or not. I guess I'm trying to say, there may be many reasons the AP index for two similiar schools, with similiar students, could vary based soley on the decision to 1) take an official AP class, and 2) take the tests, and then 3) be motivated enough to score at least a "3".