My understanding is that each consecutive letter becomes more permanent as you age, with the P/J axis not really solidifying until some time in your mid-thirties. But even if that's not the case, my theory is that as you learn new skills, you employ the ones that work best in each situation. When you take the MBTI, it is difficult to answer with more than one context in mind, and whatever context you decide is "dominant" at the time will prevail. Thus, you may be more ENTP, but act more ENTJ in certain situations, and answer accordingly. They're all a spectrum, anyway, not binary. For context, I have traditionally been an ENTP, but have tested with one or both the T and P switching to F and J.