One of the NetApp gurus told me that some processes are not as well threaded as others which causes the asymmetric CPU utilization. I believe it is possible to really see what is going on by doing a "priv set advanced" and then using the ps command but I haven't really tested that. I've also anecdotally been told that multi-threading is better on 7.3.2 and later.
What ONTAP version are you running? As already said in this thread: 7.3.2 onwards is dealing with multithreading better than previous versions, so if you are on anything prior to 7.3.2, ONTAP upgrade may solve the issue.