An introvert is a person who chooses to be alone. They chose to internalize and hence, their nature is a voluntary one. On the other hand, a shy person fears being around people. Their inhibitions keep them away from crowds, making their behavior involuntary. Since these behavior seem the same at a superficial level, they are used interchangeably.
It's a bit deeper than that, we don't have a need for conversation. In a recent interview she's tried to clarify how she and the ghost plan to have a child together. But I'm hoping through understanding what's going on with my body, I'll be able to.