Perchè è esattamente il comportamento previsto per il metodo hitTest, come scritto nel manuale:
codice:
myMovieClip.hitTest(x, y, shapeFlag)
Compares the x and y coordinates to the shape or bounding box of the
specified instance, according to the shapeFlag setting. If shapeFlag
is set to true, only the area actually occupied by the instance on
the Stage is evaluated, and if x and y overlap at any point, a value
of true is returned. This is useful for determining if the movie clip
is within a specified hit or hotspot area.
myMovieClip.hitTest(target)
Evaluates the bounding boxes of the target and specified instance,
and returns true if they overlap or intersect at any point.
Tradotto:
Valuta i riquadri di delimitazione del target e dell'istanza specificata, e restituisce "true" se si sovrappongono o intersecano in qualunque punto.