People are always trying to explain their behavior and feelings, and using a word like introvert or extrovert is easier than trying to make sense of all the reasons to ones behavior. I do believe that genes affect our behavior to some extent, but most people who call them selves "introvert" or "extrovert" could behave completely differently in a different time and place.

E.g. I used to think of myself as a extrovert when I was younger, then at age 16 I moved to a new country where I did not speak the language and I was really quiet for the first weeks until I started to learn some Spanish. But by then most people around me had gotten used to me not talking and started thinking of me as the quiet girl, and they basically stopped talking to me. At first I tried to explain my silent behavior according to the surroundings, that I did not talk much because I did not know the language, but with time I even started viewing myself as a introvert and therefor I started behaving as one. And my behavior has kept changing back and forth from being more social and talkative to more withdrawn depending on my surroundings and the people I am with.