Unfortunately, some sites do know whom they are going to rank ahead of time. I attended an APA med school internship last year, and the interview process was just going through the motions. The site ALWAYS ranks the graduate students who did their externship there in the top slots. I saw the backroom dealing going on the whole time. The only way prospective interns on the outside of the process become interns there is if the pre-selected applicants don't rank the med school high
At my graduate program, I also had to extern at an APA internship. The site served as a back-up internship site for students from my program. For example, if you did not want to spend the $$ to interview at out-of-state sites or if you wanted to stay in the area, you would simply tell the site's director and you knew where you were going to end up. Despite all this, the site still held interviews to outside candidates, lol!
These types of things are very uncommon, though. When I interviewed, I could tell most of the sites were still trying to make up their minds regarding who to rank. The key is that you have to make yourself stand out in a good way. The interviewers hear the same canned responses all day and, unless you have strong, unique responses, you will be forgotten the second you walk out the door..