It's true that 4.28 + 1.10 = 5.38,
and that would make sense if the backups were purely incremental, and data that no longer exist locally was deleted from the incremental backups.
But that's not the case.
I deleted the existing backup manually, and took a new one right away. That reduced the backup size from 2.6 GB to 1.3.
One problem is that when you delete an app, it's data is still left in the backup. You can erase that data from the latest backup, and I did that and took a new backup but the total data backed up was still twice what it should be. So a lot of old backed up data is obviously not deleted from iCloud, and I have searched online but haven't found a way to examine what data that is, nor a way to delete it separately from the fresh backup.
Apple has made the whole thing a blur, and there's no documentation.