You need to find out where those tapes physically are.
When you perform an Inventory, NetBackup does not go and check the slots.
NetBackup sends a message to the tape library (robot) to report back on contents of slots.
So, the Inventory is the robot telling NetBackup what is in each slot.
If you want to query the robot directly, you can use the Web interface for your tape library, or else use the robtest utility.
Open CMD window and navigate to volmgr\bin path as per this doc:
robtest is a menu utility (see above doc).
To see the contents of the slots, type the following in robtest menu
Please copy the text in the output and show us what you see.
Remember to quit out of robtest menu when you are done (robtest takes over control of the robot and NBU will not be able to use the robot for backups or restores).
About your private message about tapes that are 'not assigned' :
Media will only get 'assigned' the 1st time it is selected for backup.
One more thing - CLNU31L1 seems to be a CLEANING tape.
You need to configure it as hcart-cln media type and in the NONE pool.
The hcart media type means that NBU will believe that it is a data tape and will try to use it for backups, and then freeze the tape when error is received back from the tape drive.
There are TNs and instructions in NBU Admin Guide I on how to create barcode rules for cleaning tapes.