Why would SQL Server 2008R2 (SP1) x64 Enterprise only use 21g of 98g available????

If the same value is specified for
both min server memory and max server
memory, then once the memory allocated
to the Database Engine reaches that
value, the Database Engine stops
dynamically freeing and acquiring
memory for the buffer pool. https://technet.microsoft.com/en-us/library/ms180797(v=sql.105).aspx

Lastly, please refer to the following article showing maximum available RAM to specific SQL Server edition, but since you specified that you are running enterprise it should not be restricted.

https://msdn.microsoft.com/en-us/library/ms143685(v=sql.105).aspx

UPDATE:

Run the following script to verify currently set and running values. Value_in_use for Maximum SQL Server Memory should be equal to Target SQL Server performance counter. On my local instance it takes just under 1 minute for values to be updated.

SELECT  name
      , value
      , value_in_use
      , minimum
      , maximum
      , [description]
      , is_dynamic
      , is_advanced
FROM    sys.configurations WITH ( NOLOCK )
WHERE   name IN ( 'max server memory (MB)', 'min server memory (MB)' )
ORDER BY name
OPTION  ( RECOMPILE );
 
 
SELECT  'Target Server Memory (MB)' AS CounterName
      , cntr_value / 1024 AS CurrentValueMB
FROM    sys.dm_os_performance_counters
WHERE   counter_name = 'Target Server Memory (KB)'
 
SELECT  'Total Server Memory (MB)' AS CounterName
      , cntr_value / 1024 AS CurrentValueMB
FROM    sys.dm_os_performance_counters
WHERE   counter_name = 'Total Server Memory (KB)'