The reason is related to the "insideness" definition. A point is considered to lie inside a Shape if and only if:
- it lies completely inside the Shape boundary or
- it lies exactly on the Shape boundary and the space immediately adjacent to the point in the increasing X direction is entirely inside the boundary
- or it lies exactly on a horizontal boundary segment and the space immediately adjacent to the point in the increasing Y direction is inside the boundary.
Listening to: Take me to Church - Hozier