ActiveMQ Producer Flow Control Warning Stats Do Not Match <systemUsage> Configuration

Issue

An ActiveMQ message broker has been given the following configuration:

<systemUsage>
 <systemUsage>
    <memoryUsage>
      <memoryUsage limit="1430 mb"/>
    </memoryUsage>
    <storeUsage>
      <storeUsage limit="48 gb"/>
    </storeUsage>
    <tempUsage>
      <tempUsage limit="24 gb"/>
    </tempUsage>
  </systemUsage>
</systemUsage>

And the following producer flow control (PFC) INFO statement was being put out to the ActiveMQ log file:

2015-04-29 13:10:32,125 | INFO  | .169:42169@59176 | Queue                            | mq.broker.region.BaseDestination  638 | 96 - org.apache.activemq.activemq-osgi - 5.9.0.redhat-611423 | Usage(default:store:queue://tracking.process:store) percentUsage=99%, usage=15521908258, limit=15521861902, percentUsageMinDelta=1%;Parent:Usage(default:store) percentUsage=100%, usage=15521908258, limit=15521861902, percentUsageMinDelta=1%: Persistent store is Full, 100% of 15521861902. Stopping producer (ID:cmfusevml03.gain.tcprod.local-51778-1429019877150-3:1:5:1) to prevent flooding queue://tracking.process. See http://activemq.apache.org/producer-flow-control.html for more info (blocking for: 2s)

Note how the store limit of ~15gb does not match the configured storeUsage limit of 48gb. So what’s going on here?