Why doesn't "Total Space Reserved" equal the sum of Data File space reserved and Transaction Log space reserved in the Disk Usage Report?
Those files can get out of sync.  Run DBCC UPDATEUSAGE
https://docs.microsoft.com/en-us/sql/t-sql/database-console-commands/dbcc-updateusage-transact-sql

Do not run DBCC UPDATEUSAGE routinely. Because DBCC UPDATEUSAGE can take some time to run on large tables or databases, it should not be used only unless you suspect incorrect values are being returned by sp_spaceused.
Consider running DBCC UPDATEUSAGE routinely (for example, weekly) only if the database undergoes frequent Data Definition Language (DDL) modifications, such as CREATE, ALTER, or DROP statements.