blender not using all of CPU's power

i have just made an image and while it was rendering my computer was only using around 10-30 percent of my CPU to do it. In past renders it would usually be using 290 or so percent of my CPU. i have a mac and this info is coming from the activity monitor that is with the system. the render contains 11,457,744 vertices and my computer specs are a dual core I7 CPU and 8 gb of RAM. so my question is why is the CPU usage so low now?

Please supply important info:
What blender version ?
What renderer ?
Render settings ?
Other relevant info

Upload blend file to and give us the download link

2.67 is the version, i am using cycles render engine, the render settings are(dimensions are 1920x1080, output as a JPEG,limited global illumination, Dynamic bvh, cache bvh, save buffers, persistent images, anti-aliasing at 16 samples, the output quality is 100%, tiles are 16x16, and 2000 samples), in the user preferences under the system tab under compute device the top box is highlighted blue and says none while the box below it says CPU. I’m sorry but i cant get you the file, it is to large for the link you gave me, it is around 751 mb.