Different results from sys.dm_db_file_space_usage with database prefix
It looks like that particular DMO was changed to work with all databases. This can be demonstrated by running the following scripts on a 2008r2 server.

SELECT SUM(unallocated_extent_page_count) AS [free pages], 
(SUM(unallocated_extent_page_count)*1.0/128) AS [free space in MB]
FROM model.sys.dm_db_file_space_usage;

-- The "other" version
SELECT SUM(unallocated_extent_page_count) AS [free pages], 
(SUM(unallocated_extent_page_count)*1.0/128) AS [free space in MB]
FROM tempdb.sys.dm_db_file_space_usage;




As you can see the output will be the same. However, if you run the same scripts on a 2012 server the output will be different.



Technet explains that in the 2008R2 the DMO only worked on tempdb http://technet.microsoft.com/en-us/library/ms174412(v=sql.105).aspx and in 2012 that restriction was removed http://technet.microsoft.com/en-us/library/ms174412(v=sql.110).aspx.

Hope That Helps.