Much of the reason for smaller difference is that this particular benchmark is not a great overall test - a great deal of the render job with this scene is preparing it for the renderer itself, loading objects and texture data, and it's all done in one thread (a great pity).  Although it is good info, a more thorough benchmark would have things like reflection, AO, and the like - although Poser is still not very good at the multitasking business as I think we've discussed before, because render threads that finish early leave you with idle cores, and the work is not re-divided to make the idle cores busy again.  Depending heavily on how the scene is laid out, e.g. if you have tons of reflection in the upper right corner of the frame, having multiple cores may not be a very big benefit there.

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