Why the Ansible module yum is using /usr/bin/dnf instead of /usr/bin/yum?

Issue

• Why the Ansible module yum is using /usr/bin/dnf instead of /usr/bin/yum?

# ansible -m setup rhel6.example.com -a 'filter=ansible_pkg_mgr'
rhel6.example.com | SUCCESS => {
    "ansible_facts": {
        "ansible_pkg_mgr": "dnf",
        "discovered_interpreter_python": "/usr/bin/python"
    },
    "changed": false
}