>Example: V1 of a playbook installs htop. V2 doesn't. Now, a machine that went through V1 then V2 has a different state than a machine that just went through V2

>Example: V1 of a playbook installs htop. V2 doesn't. Now, a machine that went through V1 then V2 has a different state than a machine that just went through V2. This has nothing to do with idempotence. Idempotence means you can apply the same action more than once, and all applications after the first are no-ops. In Ansible's case, that applies to its tasks; eg if you run a task that starts a service but the service is already started, the task simply succeeds without any other effect, as opposed to failing or some other behavior.