"Half-past eleven," Ron muttered at last, "we'd better go."
They pulled on their bathrobes, picked up their wands, and crept across the tower room, down the spiral staircase, and into the Gryffindor common room. A few embers were still glowing in the fireplace, turning all the armchairs into hunched black shadows.
(Harry Potter and the Sorcerer's Stone)
Why is there ‘shadow’ instead of silhouette or contour?