[edding4500]

I would assume that extroverts, trying to obtain gratification from outside themselves, boast out with ideas, opinions and the like much more easily than introverts and are thus perceived as more confident. Confident as in "I am confident in myself, hence I say this thing out loud". An introvert may say that thing out in his/her head and leave it at that, so no perception can be made from outside.

[moffkalast]

And on the other hand if you're an unconfident extrovert it would also keep you from boasting with ideas out loud. As both of these cases look mostly the same from the outside, and I would speculate it leads this sort of introvert == unconfident general view.