PsiElement.firstChild vs PsiElement.children.first()

I expected PsiElement.firstChild and PsiElement.children.first() to always return the same PsiElement, however, it turns out that (considering KtElements):

• for KtBlockExpression they are not the same: firstChild returns a left curly brace and children.first() returns the first expression of the body.
• for KtTypeReference they are the same thing.

So the questions is:

• Why are they not equivalent in general?
• When are the equivalent and when are they different (I am interested mainly in classes derived from KtElement)?