The more you morph a figure, the more the rigging will degrade. So the closer you stay to the default shape, the better the figure will still bend. Genesis is androgynous, so it has to morph quite a bit to become either V5 or M5. G2F is female, so it only has to morph a little to become V6. Same with G2M and M6. Hence M6 and V6 bend better, because their rigging degrades less.