Your type, due to to the nature of the tests that are out there, is largely due to your perceptions of yourself at the time that you take the test.
Therefore, it seems like a type isn't of much help unless how you view yourself is stable.
In addition, a lot of the tests are phrased in a yes or no format when it is quite possible that both options apply to you fairly equally (Which is the case with me, especially when asked it comes to the feeling or thinking questions).
So how on earth are you supposed to know if you have the right type if you alternate between several, and none of them fit you perfectly?
I honestly don't know if anyone will be able to give an answer to this question, but it is the main question that plagues me about the mbti, and the reason I list my type as Ixxx...