I think sometimes we change as we grow and experience more--sometimes we really settle in more on what we always knew we were, and then sometimes we realize that what we thought we were before was due to wanting to be seen in a certain light. It's individual, I guess. An ISFJ friend of mine tested as INFP in high school because she had this ideal at the time of being the kind of person who believes in standing up for something and affecting change. When she read those descriptions years later, she realized that while she has those qualities, the diagnosis was really more "this is how I wanted to see myself."
And you could be closer to the middle of the road on the T/F continuum.