OK, I think I get part of the reasoning, here. The main thing is that it's an equation that is true based on an equilibrium condition. When the condition isn't true, then the theory doesn't cover the case.
Still, I see no satisfactory answer to the OP's question. What is it about Fi/Te and Fe/Ti, as pairs that make them the same or different? In other words, in what way is an INTP like an INFJ, and are they usually similar in these ways? How is an INTJ like an INFP and vice versa? And like the OP, I'm not asking this in terms of MBTI, which is something that none of the respondents to this thread so far seem to get, except maybe the ones who acknowledge they have no answer other than that's how it is. (Using INxx as just a sample template, for familiarity, and because I suspect SJ and SP might get in the way of isolating Te/Ti/Fe/Fi.)
What is it about humanity that makes these typical arrangements for the human psyche? If I start really focusing on using Ti, will I develop echoes of using Fe? Does Fe just "make more sense" to one using Ti? How and in what ways? Similarly, does using Fe cause Ti to start working in the background, however subtly, and why might we expect that (for reasons other than MBTI theory)?
I am hoping that my questions evoke answers that are applicable to the OP's question.