Well, one thing I understand from reading lots about mental disorders and what not is that lots of "symptoms" overlap with other disorders, for example, it's quite possible you're going to get higher on the borderline personality disorder when you've got bipolar especially since I find some of the questions could be a bit more specific. Also, it's important to remember that a certain degree of "symptoms" appear in the "normal" population. Problems occur when these "symptoms" are having a detrimental effect on your life.

Also, it's obviously not a diagnostic tool. That's why it suggests talking to your doctor, going a bit more in depth and what not. I know well enough I'm not manic, but it tells me to talk to the doctor who would go into far more detail about "symptoms" than a multiple choice test could. Also, the answers are a bit subjective too, memory errors and the lack of a definition of usual or often or whatever can lead to errors too. If you're anxious about your mental health you may answer higher than someone who is not worried about it even though that other person might actually be doing worse, so really it's an okay tool for suggesting that you see someone or look deeper into an issue, but definitely don't think "Oh no! I scored high on X disorder therefore I have it" or "I scored really low on depression, therefore I must be happy". It's a self reported online test (impersonal, lacking observations), multiple choice with some flawed questions and when you take it alone, you're not getting the kind of results you would when a person trained in diagnosing mental issues is asking you similar lines of questioning ie. it's less accurate than a in person psychiatric exam for sure.