Here's my understanding from my study of the MBTI and from my own personal experience.
As an Introvert I'm forced to work as an E when engaging with other people. In group situations I'm sure that I behave as an ENTP and come across as quite likeable and gregarious. However when left to my own ends I will default to INTJ which is a lot less pleasant for third parties.
I managed to develop my Extrovert personality, mainly in a work environment, very well. However Extroverts do not have the same pressure or necessity to develop their introvert side.
I would expect most Introverts to have a well developed Extrovert side but that many extroverts do not develop their introvert side quite so well and may appear shallow.