total server memory - what is the most accurate way to get it through sql server?

How is it that the values of these 2 queries disagree between each other:

SELECT [server memory] = physical_memory_in_bytes /1024.00/1024.00/1024.00
FROM sys.dm_os_sys_info;



SELECT object_name, cntr_value 
  FROM sys.dm_os_performance_counters
  WHERE counter_name = 'Total Server Memory (KB)';

they should both be showing the total server memory.

why do they disagree?