This is interesting, I would have thought you're more extroverted than me, but no. However, your high score on openness may explain it. With openness and extroversion there's maybe some overlap, as there's openness to humans as well as other things. The difference would be in the motivation: with a higher extroversion it's more about social interaction as such, whereas with high openness it's about social curiosity.
I quite like going out to meet people when I can, but a forum doesn't give me enough of the experience I need, as there's too much missing in textual communication. Lower openness would mean that I may be more selective about whom I meet, despite being relatively outgoing. So maybe I've perceived you to be more extroverted because you're more curious about people, but when it comes to craving for stimulation from the outside world, I'm slightly higher in that respect. I just express it elsewhere rather than online.
Of course this is post hoc speculation, so maybe I'm just misinterpreting the nature of the traits to fit the results.