Because with the fps unlock activated you are disabling vsync. With vsync on if you drop below 30 it will lock at 15 until you get out of the troublesome area. With vsync off the frame rate can be anything, so that same drop that brought you down to 15fps might only take you to 25fps.
ah. Can't you get banned from Xbox Live by using it though? That's the main reason why I've shied away from enabling it.