You have noticed that the number of Inventory Solution licenses consumed exceeds the expected number of active devices.
Inventory Solution 8.x
Unmanaged computers remain in an active state. To release an inventory license, these unmanaged computers must have an unmanaged status, such as Retired.
Per the Altiris Licensing - Technical FAQ set the unmanaged computers as desired to a non-active status.
As an example way to do this in bulk, follow the steps below to set all desired unmanaged computers to Retired