Perhaps you worked on a team as a Junior Administrator but put in so much effort, hard work and contributed the best ideas that you were essentially running the project. You were managing the whole project, not by name, but you carried out the same duties as a typical Manager would. While this may have been the case, including 'Project Manager' on your CV, when it wasn't your actual title, will only come back to bite you. Your contribution should come across both in your description at interview stage and through positive references.