msdb system table fragmentation
The root cause is those tables in msdb  (e.g. backupmediafamily, backupset, backupfile) 
are MS Shipped Object. by default, Ola skip those MS Shipped object for rebuild/reorganize. 

MS Shipped Object means the objects that are created by internal SQL Server components.

Solution: Add 1 more parameter @MSShippedObjects = 'Y' to enable the Ola command to get those MS Shipped Object to defragment.