Obviously df is accurate because it checks the real space used on the filesystem. UrBackup also uses this value for its daily cleanup.
The interface shows another thing: the sum of all backups. This is a different measurement, especially if using copy-on-write. For instance, my interface shows 34TB which is insane, whereas df shows only 2TB in use.
Your case seems to be the opposite: the graph shows a lower number than df. If you’re not using copy-on-write and believe the interface has miscalculated the sum of your backups, you can click “Recalculate statistics” on that page.
Maybe there is other data on this disk apart from UrBackup?