Well, yeah, the problem is that people don't generally fit neatly into categories. Even the people making the tests know that (that's why the full version gives you a "scale" rather than a simple number).
For example, I am generally INTP, like you, but sometimes I am very definitely ruled by emotions, even to the point of ignoring facts on occasion. In particular most of my explosions on BL and the arguments I had with the admins come more from that side, and the way I talk about people like Altima and DP is also very emotional rather than logical. Similarly, I usually fit with being introverted, but sometimes I can be more outgoing, and I can also sometimes be judgemental.
I would say you are like me. Generally INTP, but if you're stressed out or unusually emotional you end up more on the feelings side.
Your basic point is right, though. Whilst the test is useful as a basis, it is not as simple as placing people in a particular category and assuming everyone in that category is the same. Two people in the same category can be radically different, whereas two people who are in slightly different categories can actually be very similar. It's also not always even obvious which category characters fit in.
The main point of doing an actually good "which character are you?" test is to leave out the stupid questions like "what hair colour would you like to have?", and to not associate characters with abilities they use but which don't really match their personality. For example, picking "darkness" as your favourite element in the first test makes you closer to Sakura, because that's the element she uses, but personality-wise she doesn't fit with that at all. Similarly, my favourite colour says relatively little about my personality (although it does say a certain amount about my political beliefs), and certainly doesn't imply I match up to Rin just because I also like the colour red.