OpenCV: MatchShapes totally wrong

I'm using the cv2.matchShapes() function in OpenCV to find the shape in an image most similar to another shape.

It's giving me some weird results, e.g. when I match the shape (a circular coin) against the shape of a sweater it returns 0.09, a close score, and a better score than when it matches against an actual coin.

This is my code:

    for contour in cnts:

    box = bounding_box(contour)
    orig = image.copy()
    cv2.drawContours(orig, [contour, reference_contour],
            -1, (0, 0, 255), 2)
    cv2.putText(
            orig, "SIMILARITY: {0:.4f}".format(
                cv2.matchShapes(
                    contour, reference_contour, 1, 0.0)
                ),
            (10, 70), cv2.FONT_HERSHEY_SIMPLEX, 0.65, (255, 0, 0), 2)
    cv2.imshow("coin_metric_cnn.py", orig)
    cv2.waitKey(0)

Am I doing something wrong?