That's odd. Mine shows up with the correct addition. It seems like it's not registering the iCloud Photo Library portion.
I'd probably start with deleting backups first, to see what it does. Then just back them up again, depending on the results. There's a part of me that thinks turning off iCloud Photo Library and re-enabling it would be the better choice, since that seems to be the amount that's not being factored in, but that makes me nervous since you don't know which photos are on the device vs the cloud. Maybe someone who has done it would be able to provide some info.