The person's goals may not be as clear - he may have suffered failure or trauma in the past and therefore have less confidence and energy. These are examples of where the introversion is caused by psychological barriers or problems, i.e. there is a degree of neurosis. But introversion is also a natural part of the cycle of contemplation, analysing situations, thinking about plans and goals, etc, which then leads on to the active expression of decisions that have been made, which is the more extravert side of behaviour. This is described in Reversal Theory in 'Transforming the Mind' and corresponds to paratelic and telic states - reach and withdraw, involvement and contemplation. Where these things are inhibited or compulsive, you have problems.
High scorers tend to be volatile and demonstrative.
Those with low scores inhibit the expression of their thoughts and feelings, and try to bury upsets in the past, leaving themselves frustrated and unheard. I'm sorry if I give the impression that extraversion is a preferable state to introversion, that is not intended. Gone are the days of the strong silent hero. Kids want to be rock stars or movie stars and live for the glitter and the feel of the spotlight and who can blame them when all of society is telling them that extroversion is best. The quiet introspective sort doesn't get the same attention as the other kids.