I scored as INFP in my adolescence too!!! And INFJ once or twice.
I think it really just had to do with the fact that I was young and idealistic. I was too sheltered to see anything but the good in everyone, and I always gave the benefit of the doubt. The older I got, though, the more I realized that INFP just didn't fit. I saw a pattern of subtle differences between me and every INFP I knew. For example, they were always much better at socializing and seemed less awkward or "off" than I did, but I was always much more aware of what was acceptable and what was not. The INFPs I knew in high school often had a lot of friends, but would also say things like "I hate literally everyone" in a room full of people, or loudly talk about the really private details of their personal lives. On the other hand, I was definitely a loner, but I was much more observant of social rules and norms. Pretty much everything I did was a way of avoiding uncomfortable situations, the one exception being when I felt the need to comfort or stand up for someone who was upset and/or being treated unfairly (Fe, but I originally thought it was Fi).
This balance of awkwardness and awareness led me to the conclusion that I was either an INTP or an INTJ, and I know I'm too disorganized to be an INTJ.