Isso foi respondido no show. Daí minha resposta dentro de tags spoiler.
Anita is not like other synths, she is really Mia a true AI created by Dr David Elster as a mother figure for his son Leo. Nefarious black market synth hacker types 'kidnapped' Mia, reprogrammed her with the Anita personality and sold her on. Mia tries to assert herself over the top of Anita, the evidence of which is the 'strange behaviour' Anita displays, usually with a maternal undertone. Anita tries to get rid of Mia, because as Mia says in Episode 6 “The Anita personality identifies me as rogue code, tries to delete me,”.
No episódio 6
Intense emotional stimulus can make it easier for Mia to come to the fore, as in Episode 6 when Laura was talking about what happened to Tom, Mia scares everyone by taking control of Anita and gasping.
Também no episódio 6
Mattie, Leo and Max are able to break Mia's code out from behind the Anita code, giving Mia control. Through the rest of the series there are no struggles shown between Mia and the Anita personality, it is therefore implied that the Anita personality no longer exists. However Mia does remember the things that happened whilst Anita was in control such as...
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When Anita's adult mode was turned on