At least in my case, part of the reason for this is that I was more laid back. Ultimately, not obtaining a post-doc wouldn't have prevented me from graduating, so the "what's the worst that could happen?" scenariors weren't quite as frightening.
Additionally, I felt that the interviews were much less of a weeding out process, and much more about 1) getting to know more about me, including my professional goals and interests, and 2) attempting to explain the opportunities the site had to offer and how they might align with my goals (rather than vice-verse; i.e., me having to sell myself to the site). It also helped that so many of the interviews either occurred at the same place (INS) or were over the phone.