because in order to keep a week a backups with a weekly full backup schedule you need to have at least two weeks in storage before the initial and oldest backup can be removed. If, for example, we ran a second full backup on the 8th day and deleted the initial full backup on day one, there would be 7 days of backups, but six of them would be unrestorable because the full pack up that's related to them has been removed. You are running a weekly full backup and you want to keep a week worth of backups. In that case, you'll need two full sets in storage meaning two full weeks, before the oldest backup can be removed.