It is only for the display part. To account the current snapshot we are reducing the retention count by one.
Like in the above example mentioned, the retention logic doesn't consider the snapshot taken in the current workflow. Ideally there are 13 snapshots(it is shown above as 12), but as it is not considering the current snapshot, the retention number is also shown by reducing one.
There is no issue in the retention logic. But if the message is misleading, we can correct it.