SQL Server 2012 enlarge database space
Referring to DBCC UPDATEUSAGE (Transact-SQL):

Update the stats - sysviews are out of sync.

It seems the system views are not always accurate, and sp_spaceused is notorious for this. A form of caching is applied to non-critical, but heavy process data collection functions - like measuring partitions (like what you just created another of) and allocations.

Examining sp_spaceused (Transact-SQL), we see this: 


  When updateusage is specified, the SQL Server Database Engine scans the data pages in the database and makes any required corrections to the sys.allocation_units and sys.partitions catalog views regarding the storage space used by each table. There are some situations, for example, after an index is dropped, when the space information for the table may not be current. updateusage can take some time to run on large tables or databases. Use updateusage only when you suspect incorrect values are being returned and when the process will not have an adverse effect on other users or processes in the database. If preferred, DBCC UPDATEUSAGE can be run separately.


Try running:

EXEC sp_spaceused @updateusage = N'TRUE';