Stomp inactivity timeout takes significantly longer than specified

When I specify a timeout using a connector like the following to get a 5-minute idle timeout

<transportConnector name="stomp" uri="stomp://0.0.0.0:61613?transport.defaultHeartBeat=300000,0"/>

It takes 15 minutes for it to time out. I tried one previously at 900000 (15 minutes), and it had not timed out after 35 minutes, when I had to leave.

So, it appears the timeout takes 3 times longer than specified. I find it suspicious that there is a call somewhere I saw in the code that divides the timeout period by 3.