Hi. I don't think your interview is an error at all, and you should not look at the results of this process that way. Departments at various schools look for different things in applicants, and so is the case that WashU liked you. Remember that clinical psychology is extremely competitive, and applications have gone up in the last few years. There are also a LOT of factors that affect whether or not you get an interview or an acceptance (fit, whether or not the POI is accepting a student, funding, etc.). I don't know how your whole application looks like but if you think there is room for improvement (GRE, publications, presentations, etc.) then I suggest taking the time to work on them to improve your chances for next year.