OK. Some things needed clarification.
First, your problem isn't specifically a problem with Tomcat, it's with a multicast notification system named "JGroups". (
http://jgroups.org)
Second, this isn't a cluster in the accepted sense, it's a notification group. I think that the fact that one of their examples said "Cluster" confused you.
What I suspect is that your Tomcat server and your JBoss servers are doing is that they're using different mechanisms to construct and host their channels. For example, the JBoss implementations might be layering themselves over JMS, which isn't part of Tomcat. So Tomcat might be using a raw socket listener or something other than JMS.
That's about as far as I can go, though. At that point it requires detailed knowledge of how JGroups works.
The secret of how to be miserable is to constantly expect things are going to happen the way that they are "supposed" to happen.
You can have faith, which carries the understanding that you may be disappointed. Then there's being a willfully-blind idiot, which virtually guarantees it.