For people who don't understand, I'll be specific. The MBTI Inventory asks questions about your preferences. All it measures is your responses to the questions. If your answers change, your result changes.
The MBTI will give you the same Type code every time if you answer the questions the same way. Millions of people do answer the questions the same way. "It should be understood that the MBTI® instrument meets and exceeds the standards for psychological instruments in terms of its reliability. (ref)".
Perhaps because it involves humans, however, many people attribute "different results" for the MBTI to a fault in the assessment ("it asks bad questions!"). The fault obviously cannot lie with the person answering, i.e. "I can't possibly be inconsistent in my understanding of my internal preferences!"
If we make an analogy between the MBTI and a thermometer, your answers are analogous to a cup of tea. The measured temperature of the tea can vary, depending on when it's poured, the temperature of the room, the type of cup, how long ago the tea was poured, the temperature of the water used, and many other factors.
If you think that you can use the same thermometer on many cups of tea and always get the same result, you're not thinking clearly. This only works if you very carefully regulate the properties of the cups of tea.
If your MBTI results change, that means that the "you" answering the questions today is not responding the same way as the "you" answering the identical questions six months (or whenever) ago. If your result changes, you should not claim that the MBTI is unreliable. Instead, the much more interesting question you should ask is, "why did my answers change?