How can siblings be raised the same but turn out totally different?

[Chainfire]

6. Children are treated differently by parents.

I had three siblings. There was a clear favorite and a clear "black sheep," and the two of us in the middle. The favorite had a financially successful and apparently happy life. The least favorite, although the smartest of the bunch, by far, has had a miserable existence with emotional issues her whole life. She will die broke and friendless. You can grow up, under the same roof, with the same parents, and have a totally different childhood.

My mother also tried to treat my two children differently, she clearly favored my son over my daughter. She broke my daughter's heart with the unfair treatment, so I ended up severing our relationship with her when my children were very young. I don't think that my mother ever realized why she was cut out of our life and I had no desire to explain it to her.