When managing groups and their membership on Windows, the output of the Puppet run is misleading.
Base on that output, you'd think that Puppet just changed the Administrators group to contain only the members bob and sally. But you'd be wrong! Puppet just added bob and sally to the group.
This is the code that was run to generate the screenshot:
I'd expect that setting auth_membership => true would show the output from the screenshot. And I'd expect that auth_membership => false would show the two new members being added to the existing members, not replacing them.