I appreciate the vote of confidence!
Broadly speaking, I applied under region X to work with an expert on region X. My primary interests fit solidly within region X. But, I have secondary interests that involve the relationship between region X and region Y. So what if in three years I find myself more on the region Y side of things? Or what if my secondary interests become my primary interests? Is this the kind of thing that happens all the time in graduate programs, and do professors expect this to happen? I don't want to go to a program and have my advisor expect me to do research on particular topics and then gradually shift away from those (as I take coursework in the first couple of years, etc) and then run into problems with my advisor being mad or something. Or maybe this is completely irrational!