I think thats just how it worked. Only uses 3/4 cores on my system, but its all about GPU cores, im sure your graphics card is a lot better at crunching than your CPU?
If you run Pyrit im sure you will notice lag when trying to do anything else. I imagine using all the CPU cores would make the system to unstable.
A quote from a webpage i Googled, its using a different program but still Cuda.
"One core is always used to keep track of what the GPU does."
Cuda not using all my proc cores