Probably because they were all doodled after the film, not during (I imagine it would be too dark).
It was probably only shown for a few seconds, not long enough for everyone to get an exact idea of it and commit it to memory. One person incorrectly remembers it with a jagged mouth, perhaps; maybe another person thought they remembered it having foliage on its hands, and still another person doesn't remember any hands at all. The result is what we have now: A bunch of varying designs that all still have some resemblance to one another. d:
