For one, most people forget that Introversion/Extroversion is about more than desire for social interaction with lots of people. Energy levels and tendency to experience positive emotions are both part of the picture. One can be relatively social/gregarious and still mostly introverted.
It's hard to imagine a shy extrovert, but there must be at least a minority of extroverts that experience some degree of social anxiety (perhaps an extrovert with high neuroticism?). On the other hand, an introvert can be less interested in being sociable without necessarily being anxious about interacting with people.
There is, of course, overlap (if both introverts and extroverts can be shy, there's no getting around having some shy introverts). A majority of shy people also probably consists of introverts. That may or may not mean that the majority of introverts also experience social anxiety.