take the INFP/ENFP comparison. with an INFP, Fi is their dominant function. now whenever i see Fi in any type (as a dominant function especially) it tends to show itself in an almost negative light. its almost dark. but that is just how it shows itself. is it really dark, of course not. that is why with the ENFP you don't see that "darkness" because it filters through the light of extroverted intuition (thats a vague explaination).
I also notice that introverts might not show it as much because (a direct quote from a close friend) "it takes too much out of me (them), if it is something i (they) can handle then what is the point"