A person with a purple shirt is painting an image of a woman on a white wall.
A woman paints a portrait of her best friend.
A person is not necessarily a woman.  An image is not necessarily a portrait.  A woman is not necessarily her best friend.
A person with a purple shirt  painting an image of a woman on a white wall doesn't mean that person is painting portrait of her best friend.
Person doesn't necessarily mean a woman.  An Image of a person doesn't have to be a portrait of said person.  the woman beign painted doesn't have to be the best friend.