Lightmap pack makes sure that no face overlaps and that each face is (mostly) scaled in proportion to it's real size and is undistorted. It also leaves a margin between faces.
Adjacent faces if the mesh face are smooth (which this example is) don't need a margin because the normal changes smoothly anyway. Also, a small amount of distortion is tolerable. Thus it make senses to pack regions of nearby faces together rather than make all faces alone. Is there an easy way to do this?