MBTI results depend on the answers to its test questions. It's not too hard to get an accurate description from this if you've studied in-depth psychology enough.
True, but then you would have to a pretty manipulative sort of person to want to tweak the results to show you as something you are not.
Much as I would love to be my shadow type (total opposite) and much as I know what answers would achieve that result for me, the end result would be a lie, and I can't stand lying to myself.
That's why the results aren't tweaked and it is used as a reliable test. MBTI predicts the predictable. Personality test results never surprise me.
It wouldn't be too difficult to turn you into someone you did not think you would be, have you heard of Milgram's experiment? It's quite interesting and shows the darker side of humans, I know many had to go through therapy after it as they were so shocked by their results.