The value of Cpk is calculated by finding the minimum of:
If the process is centred exactly on the nominal, then both values will be equal, and they will be the same as the value of Cp. In practice, however, this is unlikely to be the case. If the first value is smaller, then that means that the process mean is closer to the upper specification limit, and if the second value is smaller, then the process mean is closer to the lower specification limit.