Basically, because some people have little understanding of themselves, they receive different results, and somehow conclude that this is a failure on the part of the test. It isn't. It's a personality test that categorizes you according to your "preferences" and ascribes a specific type. It can only do this if you answer honestly. The test also recommends that you read through the types and select which type best describes you, keeping in mind your results. All surveys are ultimately plagued by response bias; it's due to the fact that most people are stupid or ignorant, or both, and many are simply liars.
I have tested as an INTJ from the very beginning, before I even knew of or understood the basics of MBTI, and have retested as an INTJ every single time. I was very surprised by how well the INTJ type seemed to reflect me.
People, you are not so unique that you cannot be categorized.