Topic messages enqueued X but dequeued is less than the X

Having a simple topic created for 1 consumer subscription, sending X no. of
messages from producer but only (X-y) no. of messages are getting consumed
in the onMessage(). The admin console attached below:

Have tried to acknowledge using explicitly client ack also played around the
setting in the activemq.xml file too: some changes done

<pendingMessageLimitStrategy>
<constantPendingMessageLimitStrategy limit="50"/>
</pendingMessageLimitStrategy>

<systemUsage>
<systemUsage>
<memoryUsage>
<memoryUsage limit="64 mb"/>
</memoryUsage>
<storeUsage>
<storeUsage limit="100 gb"/>
</storeUsage>
<tempUsage>
<tempUsage limit="10 gb"/>
</tempUsage>
</systemUsage>
</systemUsage>

<transportConnectors>

<transportConnector name="stomp"
uri="tcp://localhost:61616?maximumConnections=1000橪;wireformat.maxFrameSize=104857600"/>
<transportConnector name="amqp"
uri="amqp://localhost:5672?maximumConnections=1000橪;wireformat.maxFrameSize=104857600"/>
</transportConnectors>

Not sure where the message are getting lost! Some runs give the desired
result - no. of messages enqueued = no. of messages dequeued. And have the
processing done right in the onMessage().

Any suggestions / thoughts here is appreciated. (Hope i have articulated
well to understand the issue i am facing)..

Thanks in Advance :)
Maurya