Scheduled incremental disk backup changes itself to differential backup (Backupper Pro)

As the discussion title says, I have a scheduled incremental disk backup that changes itself to differential backup.  I noticed this had happened before and thought I may have selected the wrong scheme, so I deleted it and created a new one making sure I specified an incremental backup.  I did this yesterday scheduled it and ran the backup.  However, when I checked today's backup, it again was changed to a differential backup, and for some reason it created a full backup.  I've never seen this before on my 32 bit Windows 7 machine, but this backup is running on a 64 bit Windows 10 machine.  Both machines are running the same verion of Backupper Pro.  I'm also having an issue with a system backup that is running on the Windows 10 machine, and I'll post that in a new discussion.