You know that Machokes need to be traded to be evolved, right? However, the Machoke must feel very dejected from being given away by its Original Trainer. So, why does it grow an extra pair of arms? The answer is, it wants to impress its new owner. This is because it doesn't want to be given away again. Next time you trade a Machoke to a friend, ask him to pass the Machamp back. Or, even better. Don't pass the Machoke at all.