Need help with testing definedness using 'when_unset' The key question. Why does when_unset not catch a variable as defined if it begins with a '$'?

playbook.yml

---
- name: webserver plays
  hosts: all
  user: nathan
  sudo: no
  gather_facts: false

  vars:
    foo: onetwothree
    bar: fourfivesix
    err_variable_not_present: |
        Required variables not present. Make sure you have ~/.wa-dev-ansible-vars defined
        Use playbooks/group_vars/default as a starting point

  pre_tasks:
    - name: "ensure variables"
      fail: >
        msg="{{ err_variable_not_present }}"
      when_unset: $item
      with_items:
        - $foo
        - $bar
        

playbook2.yml

---
- name: webserver plays
  hosts: all
  user: nathan
  sudo: no
  gather_facts: false

  vars:
    foo: $onetwothree
    bar: fourfivesix
    err_variable_not_present: |
        Required variables not present. Make sure you have ~/.wa-dev-ansible-vars defined
        Use playbooks/group_vars/default as a starting point

  pre_tasks:
    - name: "ensure variables"
      fail: >
        msg="{{ err_variable_not_present }}"
      when_unset: $item
      with_items:
        - $foo
        - $bar
        

result1

[auto-1115][~/code/infrastructure/wa-dev-vagrants] ansible-playbook -i fake-inventory playbooks/tmp.yml

PLAY [webserver plays] ******************************************************** 

TASK: [ensure variables] ****************************************************** 
skipping: [localhost] => (item=onetwothree)
skipping: [localhost] => (item=fourfivesix)

PLAY RECAP ******************************************************************** 
localhost                  : ok=1    changed=0    unreachable=0    failed=0   

result2

[auto-1115][~/code/infrastructure/wa-dev-vagrants] ansible-playbook -i fake-inventory playbooks/tmp.yml

PLAY [webserver plays] ******************************************************** 

TASK: [ensure variables] ****************************************************** 
failed: [localhost] => (item=$onetwothree) => {"failed": true, "item": "$onetwothree"}
msg: Required variables not present. Make sure you have ~/.wa-dev-ansible-vars defined
Use playbooks/group_vars/default as a starting point

skipping: [localhost] => (item=fourfivesix)

FATAL: all hosts have already failed -- aborting

PLAY RECAP ******************************************************************** 
           to retry, use: --limit @/var/tmp/ansible/tmp.retry

localhost                  : ok=0    changed=0    unreachable=0    failed=1