Ansible list syntax - apparently confusing behaviour

hosts

localhost ansible_connection=local

playbook.yml

# Run this with >$ ansible-playbook playbook.yml -i hosts

- hosts: all
  vars:
    simple_list:        [one, two, three]  # lists like this are fine in yaml

    derived_list:       "[{% for thing in simple_list %}{{ thing }},{% endfor %}]"
    # Single quotes below should be unecessary accoring to my understanding
    derived_list2:      "[{% for thing in simple_list %}'{{ thing }}',{% endfor %}]"

  tasks:
  - debug: var=simple_list     # a list, as expected.
  - debug: var=derived_list    # a string.  Eh?
  - debug: var=derived_list2   # a list.  The single quotes must be important somehow.