Multiple personality disorder not being a demonic possession?

If indeed abuse triggered multiple personality disorder and not by any evil spirits to help the victim cope afterwards, one must question the real life scenario of Sybil. She had one personality in which could speak fluent French and one who could play Mozart and the like on the piano. However, Sybil could not do any of these things in her own default personality. It doesn't quite make sense that the brain could compensate our misfortunes through extraordinary abilities that we have never learned before and only exhibiting them after trauma and not just because. If I'm not being clear the only way I can explain it is how can the other personalities do things we have never learned before, which seems quite impossible on our own, without being able to do it in our ideal state of mind where no disorder is present?