Difference size DBCC Page and DBCC Fileheader
Restart SQL Server and see if they suddenly match.

There is a documented behaviour where if tempdb grows you can get different results depending on how you query what size it is. For example sys.master_files shows tempdb size on start; sys.database_files shows actual size (as do some of the Fileproperty() function results).

Nacho Alonso Portillo wrote on MSDN:

Well, it happens that when SQL Server autogrows a file which is part 
of TEMPDB, the size change is not reflected in sys.master_files (or
sys.sysaltfiles for that matter). As per the functional specifications of
the storage engine, for TEMPDB the change in the size of one of its files
is only reflected in those system tables for explicit grows and shrinks, 
not for those triggered by the automatic mechanisms.


https://blogs.msdn.microsoft.com/ialonso/2012/10/08/inaccurate-values-for-currently-allocated-space-and-available-free-space-in-the-shrink-file-dialog-for-tempdb-only/