In case anyone comes back to this, it's because any snapshot who's retention date is out of the retention period can get removed. When the original item is out of the retention period it can also get removed.
There is no cumulation of diffs, its just diffs from the original object.
You might notice that the quoted text of my original post has also been changed on the link.
“When a retention policy is applied in backup repositories with the Snapshot-Based Retention type, Veeam Backup for Microsoft 365 removes versions of an item, but not an item itself. Data removal from backup occurs every time the restore point of an item's version in a backup file goes beyond the retention coverage. Eventually, if no more changes were made to an item, Veeam Backup for Microsoft 365 will remove all versions of an item except the latest one. The latest versions and items that were never changed stay in a backup repository with the Snapshot-Based Retention type forever.”
Doesn't this essentially prove that these are mostly not high skill positions that can't be filled but are actually just a way to get high skill labor at below market rates?