PricklyPearLvr:
I recently took an online quiz and I think it's 100% true. I answered the questions with complete honesty and I was tagged as a mutual gainer.
Online personality quizzes work via basic categorization. You only get a handful of categories to choose from. Which one you get depends on the answers you select. Each question is tied to a specific category. And the more answers you get for a specific category, the more you get sorted into that category.
Thus, if the quiz author knows the subject well and the questions are well-written, you can expect a certain level of accuracy from the quiz.