Quoted By:
It's because it was originally based on the actual cloning method, the difference being that Mew was actually birthing a modified clone of itself (which is kinda disturbing). Interestingly, this implies that Fuji and Team Rocket actually managed to catch a Mew.
Then the first movie came and they retconnected the story to make it more marketable, and made Mewtwo completely grow in a capsule from DNA found in Mew's hair.
