I've noticed this on my own install, but if you look at https://secure.phabricator.com/D7343, you'll see that the reviewer status is "Accepted Prior Diff". If you compare the accepted diff and the landed diff, there's no changes.
I think the idea of making a diff to show any changes between the accepted version and the landed version is a good idea, but in general I think it should also not create a second diff if the contents are identical (since that way if someone lands exactly what is accepted, then the revision will still show a green "Accepted" tick instead of a grey "Accepted Prior Diff" tick).