Now here's an interesting question/bit of food-for-thought/personal hypothesis for the PerC community. (In advance: this is not a "help me decipher my type" post. I am very confident in my INFJ-ness.) I'm an INFJ who has always felt much more identity with Fi than with my supposedly-secondary Fe. In cognitive functions tests, Fi frequently outranks my supposedly dominant Ni. I definitely have the strictly (and rabidly) held personal values characteristic of strong Fi, and I make moral decisions with a combination of Fi-ish gut reaction and Ti-logic (more in keeping with my own tertiary Ti).
Here's the thing about all those strictly held Fi-ish opinions, though: they're all egalitarian. Every last one. For those who don't know the definition of the word, egalitarianism is the belief that everybody is entitled to equal opportunities, and the belief that each person is worth exactly the same as each and every other person. The fact that I adhere so strongly to ideals that emphasize equal opportunity seems a bit sneaky-Fe.
That's my hypothesis, then: my extreme social introversion and general unwillingness to interact with other people appears to cognitive-functions tests as low social empathy, which fools them into ranking my Fe as underdeveloped, while my rigidly held opinions and values seem to the tests like high Fi, when actually, my most rabidly held opinions all pertain to the ways in which people ought to treat each other... meaning that the Fi the test sees is actually Fe in an exceedingly clever disguise.