I just think it's not worth trying to guess about at all. If you're interviewed by a regular professor, you can spend the whole time thinking they're not interested in you, and for what? Maybe it's true and you're not likely to get in - will that change how you interview? Maybe it's false and they just said everyone does 15 interviews and split up the names alphabetically.
If you're interviewed by the dean, you can spend the whole time thinking you're already in and the interview is just a technicality, and for what? Maybe it's true and you're already nearly in - will that change how you interview? Maybe it's false and they drew names out of a hat, or common interests, or who knows.
Just imagine everyone's the dean.
And I do think it's related to treating people differently. My personal belief is that once you try to outguess how they are ranking you, it *will* change your level of interest in them and how you conduct yourself during your interview. If you think you're not likely to get in, due to the status of your interviewer or whatever, you won't try as hard. If you feel like it's a good sign the dean is interviewing you, you'll feel like you're on the radar, so to speak, and you will try harder. People always are on their best behavior when they think they're being observed by someone important.
It's entirely possible the dean interviews the best applicants. Maybe, I dunno. It still seems like it's shooting yourself in the foot to think too much about it though.
Maybe you can separate the two. I suppose it's possible to simultaneously try and figure out the ranked importance of your interviewers, and still treat everyone around you with the exact same equal level of utmost courtesy and respect, but personally I think it'd be pretty difficult and not really worth the effort.
If you are involved in interviewing for business, you might also realize that even though it's true the boss might interview the best candidates, a candidate who has decided before the interview that he's got the job because he's being interviewed by the boss is likely to be in trouble.