Apart from resolution there is also the aspect ratio. Even if you use the stretch mode Remote Utilities honors the aspect ratio so that your remote screen, albeit stretched (or shrunk) still shows correct proportions without distortion.
You will not see the black bars only if the aspect ratio of your local computer is the same as the aspect ratio of the remote one, although their resolutions may differ. If the local resolution is higher than the remote one - stretching occurs. If the remote resolution is higher than the local one, shrinking occurs.