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Actually it fits Mewtwo more. Mew have been around for a long time and really shouldn't change.
However Mewtwo was a scientific experiment. He blew up the labs and flew off. So who is to say they were done working on him. What if they had a gene in him that was going to trigger one last mutation into an ultimate form but then he blew up the labs. So all this gene needed was time to mature and an event to trigger it to make his DNA more stable and his body more powerful.