That doesn't sound right. If you run a sysstat -M from the node shell or 7-mode (requires diag), it should show 8 CPU cores IIRC on the 6280. The 6290 has 12 cores.
sysstat -m would show you from CLI/node shell. I think the highest I've seen is 12 cores. It doesn't count hyperthreaded cores as separate but shows physical.