Each PI and program is different, so there isn't really a 1-size-fits-all answer. Since you got the interview, it suggests that the selection committee saw something(s) in your application that makes you a viable candidate. I'm not sure if you'll get the question "why don't you have more research experience" but you might get questions like what did you do after college (if you aren't applying straight out of college). It wouldn't necessarily be an issue, but since you're applying to PhD programs, it shouldn't come as a surprise when you're asked to describe what you did, possibly in detail. Also, the actual time spent in research isn't necessarily as important as what you learned and how involved you were. If I were you, I'd make sure I can explain everything that is on my CV and be able to talk about your research experience, what your lab did, what your responsibilities were and how they contributed to the projects in detail. The rule of thumb I followed was that if I can't talk about it at length, don't put it on my CV. If all you did was organize folders for one project and cannot talk for 5 minutes about what the project was about, don't put it on your CV.
I've never had anyone asked me "why should we pick you" (that doesn't mean it doesn't happen though), I have been asked about statistics, a certain research paradigm/methodology, and even what I think my dissertation would be and what I expect to find. I've also had interviews where I just chatted about the program.
Edit: I'm a grad student that has helped my program with the interview process. So by no means an authority on this subject.