DBCC UPDATEUSAGE (dbname) runs successfully but does not fix the "disk usage report"
These numbers on the disk usage report look correct. It's important to note that the Total Space Reserved is displayed in GB, using the conversion 1024 MB = 1 GB

The formula used by the above report is straightforward:

Data File Space Reserved + Transaction Log Space Reserved = Total Disk Space Reserved

279,860.56 MB + 90.63 MB = 279,951.19 MB

279,951.19 MB / 1024 = 273.39 GB