Take a look at the documentation on IBM’s web site: Processor Value Unit (PVU)
In general, you will purchase entitlements for the lesser of the two values - Full Capacity or Sub-capacity - they don’t get added together.
Full capacity counts all the cores in the underlying physical server and multiplies it by the appropriate value from the PVU table for the processor model used by the physical server.
Sub-capacity counts the cores assigned to the VM and multiplies it by the appropriate value from the PVU table for the host’s processor model.
If your manually calculated numbers are lower than those provided by BFI, it is likely that you need to verify that the data on the Software Classification report is valid. Are the components discovered by BFI assigned to the correct products? Should some software components be suppressed? Should some software products be excluded? Are the license metrics associated with the software products correct for your entitlements?