I read a while back that for Introverts, the J/P axis isn't clearly differentiated, while for Extroverts, F/T is unclear. So it's common to slip between INTJ and *P.

If the test has any merit at all, it's in trying to make sense of very complex patterns in an organized way that we can easily comprehend. The brain probably doesn't actively slot itself into one of 16 pidgin holes, but doing so is a useful tool for understanding.

Some of the main differences between INTJs and *Ps are that P's tend to disdain social rules as artificial constructs and doesn't think there is any good reason to follow them. J's tend to go along with social rules, if they can figure out what they are. The developer who wear's his hair a pony tail and gets into heated arguments with the boss is probably a P, while the one who just doesn't know when to shutup is probably a J.