^^ i like that explanation.
though it's kind of weird for INFP/INTP to have high Ti/Fi, because even just looking at their first four functions and discarding the rest of supposed function order:
Fi Ne Si Te (INFP)
Ti Ne Si Fe (INTP)
T and F are so low on each other's respective lists that it would be rather surprising to find it, in the opposite attitude no less, almost as high in preference as their dominant. even if we switched Si for Te/Fe it still is only tertiary, which is far from the dominant. and that's not to say it can't happen, just that it's probably rather unlikely - i would be more willing to bet that someone is mistaking Ti for Fi or vice versa than that they are actually using both Ti and Fi strongly. or perhaps even that they're ENP and having a hard time figuring it out because they have a strong tertiary.
speaking of, T/F confusion is not as weird for ENFP/ENTP, because our function order goes
Ne Fi Te Si (ENFP)
Ne Ti Fe Si (ENTP)
so not only are Fi/Ti not dominant, but Te/Fe are our tertiaries, so it's easier to see how we could develop Ti or Fi which is closer to being balanced with our auxiliary (though again i would check and make sure someone is not really just using Te/Fe and mistaking it for Ti/Fi).
by the same token, of course, it's easier to see an INP being near the boundary in N/S, as opposed to T/F. the dominant function usually is so clear to people that, unless they're simply mistaking it for the complementary same-attitude function, it's unlikely to be nearly a true ambivert on it.
of course, all of this is theoretical. i have no idea if actual statistics support it.
oh and here's a test i like -
http://www.cognitiveprocesses.com/assessment/develop_old.html. it's too obvious for some people's taste but i think it helps differentiate. the submit link is broken though - just go through and count up your tick marks.