Twins can have dramatically different experiences in life, i.e. "nurture" and the experiences you are exposed to you end up meaning that while your genetics are the same, the phenotype which is expressed is ultimately quite different. For example, one twin could be being badly bullied and hiding it, hurting their confidence and also damaging brain development due to high levels of stress hormones.
Also, there are cases where one identical twin had low birth weight due to a placenta problem, and grew up with developmental delays. Some differences really are congenital.
