The gap between "Max Server (MB)" and sys.dm_os_memory_clerks?and sum memory usage from sys.dm_os_memory_clerks?

Hello,

I have a SQL 2012 instance, i want to know is there a dmv (or from dbcc memorystatus ) i can check how sql server use memroy accurately? 

There is still a gap between "max server memory (mb)" and sum memory usage from sys.dm_os_memory_clerks,for example:

exec sp_configure 'max server memory (MB)'
-- 239000
select sum (pages_kb)/1024 as mb from sys.dm_os_memory_clerks
-- 204608

SQL Server Version:

Microsoft SQL Server 2012 - 11.0.5532.0 (X64) 
Jul 14 2014 15:00:27 
Copyright (c) Microsoft Corporation
Enterprise Edition: Core-based Licensing (64-bit) on Windows NT 6.1 <X64> (Build 7601: Service Pack 1)

The Windows box physical memory is 256gb.

Many thanks.