For the same reason human sometimes produce siamesic twins: it just happens. Something genetic goes wrong in an early stage, I think its just something that happens- but not very often- usually things doesnt survive when nature screw up, sometimes it does. I like the odd things- beautiful in all its craziness
whats interesting is to see if it does it again, like if it is in the dna or if its a random "once in a lifetime" thing.
I have no idea and no scientific reasoning behind this- just my thoughts, so dont take it to serious