Why does the Opteron 6328/6220 have two NUMA nodes?

I was under the assumption that they are a single CPU package like their 
desktop FX-8100/8300 counterparts, however under linux the 6328 is 
reported as two NUMA nodes (CPU 0-3 and 4-7) like the dual CPU 6386 
package is.

This presents a serious performance issue.

I am not sure what is reported under the stock BIOS as I am unable to 
reflash right now, does anyone know if this is a bug?

On one of my Socket C32 systems with an OEM BIOS an 8 core 4280 CPU uses 
half as much power and reports only a single NUMA node.