If load isn't being balanced correctly across your processors, here are two likely culprits:
a) Operating System
b) Java Virtual Machine implementation for said O/S
I seriously doubt there is any Jboss code that could impact processing load distribution....
JBoss relies on the JVM completely. It has no fancy code to do parallel processing with the processors on the system.
If it is a feature of the JVM(I mean parallel processing), then JBoss indirectly makes use of it.