Duplicity works by making a full backup and then series of incremental backups based on it. Eventually you can make another full backup and new series, but you cannot delete full or incremental backup without rendering whole series corrupt. In Time Machine (and - I presume - this software), if you delete a backup checkpoint for a specific date you lose only file changes captured by that checkpoint, but state of the backup at the checkpoints before and after remain intact.
I believe the correct name for what time-machine does is differential backups.
Instead of doing a full backup and then having a chain of incremental backups, each differential backup stores all the differences from the full backup, so they are much larger but it allows you to delete previous differential backups.
