my theory is that Burntrap somehow earns new parts that Scraptrap lost Burntrap also has lots of parts that are similar to the mimic's parts I think that Scraptrap actually died and parts of him were left and the mimic used them and his voice to trick Gregory that the mimic was Burntrap that is why the mimic has parts that look like Burntrap
purple mean looks the same in some ways red means vary not similar looking
