Sequential differential backups sometimes smaller in size.
I recently installed Aomei Backupper Pro 4.1.0 and am using the space disk management scheme. Sometimes the file for a differential backup is smaller then the previous differential backup. How is this possible?
In the case of AOMEI, it appears (in my experience) that differential backups use what is commonly referred to as "overwrite" or "mirror" mode, meaning that the backup data set not only includes files that have been added and/or changed but also excludes files that have been deleted on the source. Deleting a large amount of data on the source in between differential backups could absolutely result in smaller backup sizes.
Maybe windows.old, the previous version of Windows10 kept for a few weeks after a big Win10 update. After a few weeks,and you did not rollback, the windows.old is deleted automatically.
That's exactly where I was going to go. Windows 10 version 1803 update rolled out in April. It would be about due for being auto-purged now.