Maybe thinking about it this way would help us:
Te, Se, Fe, Ne as interactions (between the person and someone/something else), and since Extroverts tend to interact, these functions tend to dominate.
Ti, Si, Fi, Ni as kept within our minds (not interactions), and since Introverts tend away from interaction, these functions tend to dominate. Also, for Introverts the "Xe" functions tend not to dominate, because of the tendency away from interaction.
So when typing Extroverts, we could look at how they interact.
But when typing Introverts, we can't look at that, because that's not the focus of their world. We could look at how they mull things over in their mind.