The thing that makes me think I might not be a "complete" INTP is my lack of intrest (or proficiency) in Mathematics and Physics. Though my intrest in the Human Sciences is growing day by day (until I get bored with that too, I guess?). Not that i'm such a people person, just want to know how they function...
This doesn't matter. I've never been interested in these subjects either and I am certain that I am an INTP (well, as certain as an INTP can ever be).
I think it was on Personality Junkie where I read that an interest in humanity in the broadest sense (e.g. social sciences and the aptly named "humanities") stems, in some INTPs, from their inferior Fe.
People (generally and specifically) are systems that can be analyzed and understood too.
The thing that makes me think I might not be a "complete" ISTP is I'm not really a thrill-seeker. In fact, i'm afraid of heights and high speed car rides. The notorious hand-eye coordination isn't present either. Though I don't really enjoy having to read 50 lines of text to weed out a piece of information i'm looking for either, which as far as I can tell is not really that much of a problem for an INTP? And given the fact that i'm a musician might also somewhat pull me towards the "Artisans".
This is a little bit more consequential. The ISTPs I know definitely have the thrill seeking element from their Se.
Interests vary by person. As demonstrated by this forum, there can exist vast differences between people of the same personality type. The types are a framework for describing how people think and interact with the world and other people.
The difference between INTPs and ISTPs is that INTPs cognitive functions are Ti-Ne-Si-Fe, while ISTPs are Ti-Se-Ni-Fe. What does this mean?
The primary difference is in the auxiliary function, which in both cases are perceiving functions. They are both extraverted, so the primary difference between the two types in in how they gather information. ISTPs are about sensory details and concrete facts. They are often very mechanically inclined. My brother is an ISTP and he is brilliant with any sort of engine. His Ti-Se primary axis allows him to make correct deductions from information he gathers with his senses.
Another ISTP I know is studying neuroscience. He is still very much Ti-Se oriented, but for him it is all about empirical data that can be quantified from experiments that can be duplicated.
Then there are INTPs. INTPs gather data by looking beyond the surface (often missing tiny details) to see the connections and patterns between disparate objects and ideas. Often this manifests in a passion for fields like math or theoretical physics, but it could just as easily by philosophy or history or economics. The point is that the data is abstract, not immediately connected to some physically tangible object or phenomenon. INTPs are all about ideas.