Problem is the default parent options are merged into child options, however those may differ from the actual options, as those are not merged into children. To clarify;

$form = $this->createFormBuilder(null, ['method' => 'GET'])
  ->add('child', CheckboxType::class)
  ->getForm();

form.vars.method == 'GET'
form.child.vars.method == 'POST'

This allows for consistent access to form.root.vars.method.

I believe a BC break can happen if one named a field root. There's an issue somewhere regarding this for the parent field as well ;-)

edit: #19492 (comment)