I really don't see why INFP can't or shouldn't fit. I hope you enjoy hearing the same old boring thing... Well, I guess I could liven things up... YOU ARE AN ISTJ, YOU KNOW YOU WANT TO BE ONE... And now that I say that I realize I need more sleep so I can cage my inner ENFP.
"I am what I need to be..."
"Nemo me impune lacessit - No one provokes me with impunity."
Since feeling is introverted, it's not uncommon for an INFP come off as cool & detached & be mistaken for a thinking type. Although I don't get cool & detached from you necessarily, but Fi is obviously guarded.
Also, I've noticed some over-simplified MBTI tests write the questions which are supposed to determine T/F in a way that favors Fe over Fi. It also doesn't take into account Fi's values...such as a Fi who values what seems logical to them over being "nice" to people. For example, I've seen a test ask, "Do you think it is better to think with your heart or your head?". It doesn't ask how you think, it asks what you think is better. I think it's better to think with your head, and I view my thought process as my head, so I choose head. Then I test INTP! If I was asked whether logic or feelings color my thoughts, then I'd choose feelings.
So basically, if you have Fi and Te, then you may end up answering the question as a T, and then your results get screwed up.
"Charlotte sometimes dreams a wall around herself. But it's always with love - So much love it looks like everything else. Charlotte Sometimes - So far away, glass sealed and pretty." - The Cure