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For image backups, Reflect will only append to a set if the new backup is a "match" for the existing backups in the set. A match is defined as the backup containing exactly the same partitions, which means no partitions added or omitted, and all included partitions still having the same size and "offset" (position on disk). Windows 10 feature updates will sometimes expand the Windows Recovery partition by shrinking the C partition to create the necessary additional space. That would cause all post-update backups to no longer match the pre-update backups and thus require a new Full. This is probably the most common reason that a new set has to be created. If you want to see whether this occurred, go to the Existing Backups tab (or Restore tab if you're still on V7 even though this was posted in the V8 section), and if you select a backup, the layout of the partitions it contains will be displayed at the top of the interface. In the lower-left corner of each partition will be two numbers showing the space in use and the total size. Compare the total sizes of each partition between backups in each of your two sets.
I'm also fairly sure that the disk's unique identifier that is randomly generated when it is first initialized cannot have changed either. This unique ID is called the signature on disks initialized as MBR and the GUID on disks initialized as GPT. And on GPT disks, the individual partitions each have their own unique identifiers, and I'm pretty sure those have to remain consistent as well -- but I have not tested that specific scenario. But neither of those changes would be typical. The disk ID would change if you migrated to a new disk or for some reason completely re-initialized an existing disk, and the partition IDs would change if you completely deleted and recreated a partition, but those obviously aren't typical scenarios.
Finally, be aware that if new backups no longer match older backups, that will affect the retention policy by default, because the default retention policy "scope" is "all matching backups". So if you're using that setting, then your earlier backups would never be purged automatically. If you want to change this, change the retention policy setting to "all backups in the destination folder", which can be done directly above the retention policy settings (below the schedule). But before you do that, I'd recommend making sure that no OTHER Reflect backup jobs are sending backups to the same folder, otherwise Job A could purge completely unrelated backups created by Job B. If you don't run more than one type of Reflect backup job, then there's nothing to worry about.