Have you compared the individual functions to yourself? I pretty much use that to hunt down someone's type instead of ask them to take tests. I mean as long as you know yourself, there shouldn't be some mishaps. Before I go on rambling about INTJ functions here probably because I've been there as well, it's important to notice that once it's precised which is which, how your personality is manifested outside no longer matters.

What INTPs do most of the time is describe and verify the connection between a pure object and its possible relationships with other things; much like a computer that is fueled by Ne and Si. INTJs don't dwell on that Si, that's how I can tell an INTJ from an INTP. INTJs Ni is just like Si in that it remembers but in a blurry kind of way. Ni is concepts and memories of yourself, of your mind, being able to decipher what's at play when certain things happen. Maybe it's a visual memory, you see collages of situations or perhaps one situation out in reality but with no details, just some actions necessary for your theory to manifest. My point is, it's a memory, a continuous imagery that you don't have to pull apart and zoom on like Si with Ne. INTPs when theorizing may dive into super-categorized details, INTJs don't do that. INTJs experience Ni concepts popping out of nowhere and in flows that they are accompanied to. For example, one concept sort of pops and Te reprimands sense, engorged Fi then vents examples or values and it goes back and forth while Te maintains realistic, non-bias attitude. All the concepts dropping by can be gutted as group of close concepts but not necessarily collapsed together. INTJ goes on to do something else while the theory is being unconsciously, and suddenly the INTJ already got full explanation and a new worldview because like maybe they conceived that one concept is the relationship of two other concepts.