This is my first year applying so I don't have as much experience. I've finished 6 interviews so far and I haven't been asked this question *yet*. I have been asked "Why X school/Why our Program?" or questions along those lines. I've also been asked whether I listed X school for tri-council funding, which I guess is a one way for them to find out if it was your top 3 back when you completed tri-council applications.
Like someone mentioned, you can't answer "no, this isn't a top choice". But I'm assuming that if you've applied there, there is clearly something that draws you to the school. If it isn't necessarily your current top choice, I would maybe cling onto 1 or 2 selling points of the school/program that you genuinely do like, and/or maybe something about that PIs research that you like. But that's just my 2 cents. As someone else mentioned, top choices can change as you go meeting PIs, learning more about the program, talking to grad students, etc.