Just to clarify - i made boo-boo earlier. Programs are either single or multithreaded. However, some games just decide to throw a second thread in because they can - the game wont gain much benefit from having sound, video, AI and user input on different cores - so why bother.
So they cap it down to a certain level.
While say, sup com, COULD benefit from 8 cores, its probably only a tiny amount over 4, so they may have capped it there.
Its kinda hard to learn on this, as most 'dual core' games are simply a single core engine with a hacked-on method for using another core (the same way CoH uses a second core, but its only for physics - which are purely graphical, and dont affect gameplay at all)