It seems that if an SVG group has one element of a kind, coordinates are returned bare, but if it has multiple elements of a kind, coordinates are returned wrapped in a list. For example, one polyline is returned as [(0, 0), (1, 1)] but two polyline are returned as [[(0, 0), (1, 1)], [(0, 0), (1, 1)]].
It seems that if an SVG group has one element of a kind, coordinates are returned bare, but if it has multiple elements of a kind, coordinates are returned wrapped in a list. For example, one polyline is returned as [(0, 0), (1, 1)] but two polyline are returned as [[(0, 0), (1, 1)], [(0, 0), (1, 1)]].