Nomadic Hello Nomadic, This could be caused by something called a "backup retention". A backup retention defines how long a backup "image" is kept. The most common backup retention is known as a grandfather, father, son retention plan. In this scenario, a grandfather backup is a monthly full backup that is kept typically for a year. A weekly full backup known as the father runs every week and is typically kept for a month. And the daily backup, known as the son, is typically kept for one week.
In each scenario, the retention is overwritten after it expires. Therefore, a daily is overwritten every week. For example, if your daily ran Monday, it would be overwritten the following Monday. Your Tuesday daily would be overwritten the following Tuesday and so on and so on. The same would be for your weekly backups. If they run every Friday, then on the fourth Friday of the month, the first Friday weekly would be overwritten.
As you can imagine, this can take up a tremendous amount of space on a backup NAS. Essentially, you end up with seven daily backup images plus four weekly backup images plus 12 monthly backup images on your NAS.
Wow, I've got a headache just recalling all of this LOL. Another consideration is not just the retention plan, but the type of backup you are doing each time it runs. In other words, are you doing full backups every time a backup runs or are you doing incremental backups each time or are you doing differential backups each time? A full backup is obvious. An in incremental backup, only backs up the data that has changed since the previous backup. A differential backup contains all of the data that has changed since the last full backup. Long and short, check your backup configuration and see what the retention plan is and the type of backups that you are running.
Another final note, in the backup world, typically applications are not backed up, just the data is backed up. Applications are typically just reinstalled and or upgraded. Usually, if they're upgraded, they're backward compatible to the data from the previous application version. This is pretty typical in the business world. However, I know that the mobile application space is a little bit different.
Hope this helps.