I'm a huge introvert and the test scored me high on Extroversion.
I worked hard to acquire social skills so I could be an excellent hostess and boss. So the test seems to based on behavior rather than natural traits.
Maybe, I'd have to check the items. Your score is based on how you describe yourself, relative to how others describe themselves.
A few things to consider:
1. It's a short scale, attempting to cover the major factors of personality. That's a lot of ground. The Extraversion factor has only 10 or 12 items, which isn't much. It's just a snapshot.
2. The Big 5 concept of "Extraversion" is more multifaceted than most people's conception of introversion-extroversion. For instance, here is a list of the sub-traits that go into making up "Extraversion": Warmth/friendliness, Gregariousness, Assertiveness, Activity/Activity level, Excitement-seeking, and Positive Emotions/Cheerfulness. So "Extraversion" is a broad, abstract term that incorporates a lot of different traits (some of which you might be high on) which you might not necessarily think of, if you think just in terms of "Am I an introvert or extrovert?"
3. It's possible that you might be more extroverted than you think you are. I've known plenty of people who say they are huge introverts but actually are not; they're mild introverts. I've known other people who say they are introverts, but they're not; they are pretty balanced between the two poles (ambiverts). I'm not sure why some people like to claim to be introverts when they're not, but they do.
4. The best way to answer the questions is based on how you generally or usually are, not on how you might behave in a particular social role (e.g., as a hostess).
5. It's also possible that, because you've worked so hard to become more social, you have actually become more extroverted over time. Personality isn't entirely fixed but malleable to some degree.
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Personally, I'm always curious when I score different on a personality test than I thought I would. Sometimes it's because the test didn't perform very well. But sometimes I learn something about myself.
For instance, I've thought that I can be narcissistic at times, but when I took multiple tests that measure it, I always scored low, in one case at the very bottom. So I stopped worrying about being narcissistic and gave myself more room for self-assertion, a little healthy ego, and some pride. It was good for me.
Another example is that I used to think I had an anxious or insecure attachment style. But I took three different attachment tests over a series of several years, and I always came out securely attached. That changed the way I saw myself, and it made my relationships go better, because how you think about yourself changes things.