As to your WHY part of the question you would have to dig into the load balancer used in your Linux kernal. There were many balancers used in Linux since it rolled out and I didn't bother to keep track as it worked and I didn't need to be that close to the kernal inner workings. But that doesn't mean you can't investigate which balancer they use in your kernal.
As to the does it cost, I found more cores reduced total run or startup times so the load balancer definitely reduced costs.