115982 é uma pergunta muito semelhante
Este é um servidor de 64 bits - você tem as páginas de bloqueio na política local de memória ativada?
the most likely reason is that this SQL Server 64bit instance is using “locked pages” as I’ve described in an earlier question. Notice the name on the Task Manager column is called Memory (Private Working Set). Remember we also said that if SQL Server 64bit instances use “locked pages” this memory would not be part of the working set (because remember AWE APIs are used on 64bit to “lock” pages and that memory is not part of the working set). So since the locked pages are not part of the working set, they won’t appear in this column in Task Manager. On the Task Manager for Windows Server 2003, this column is called “Mem Usage” but it also reflects the working set of the process.
Aqui está um artigo que explica em profundidade
Você também pode ver os contadores em SQL
SELECT
object_name
,Counter_name
,cntr_value
,ROUND(( cntr_value * 8192.0 ) / 1048576, 0) AS cntr_value_MB
FROM
sys.dm_os_performance_counters
WHERE
object_Name LIKE '%Buffer Manager%'
AND RTRIM(counter_name) IN ( 'Free pages', 'Total pages',
'Database pages' )
UNION SELECT
object_name
,Counter_name
,cntr_value
,ROUND(( cntr_value / 1024 ), 0) AS cntr_value_MB
FROM
sys.dm_os_performance_counters
WHERE
counter_name IN ( 'Target Server Memory (KB)',
'Total Server Memory (KB)' )