对SPC手册第二版中一段话的疑惑

SPC手册第二版P19页第一章第一节中有一段话:

英文版:
Process capability is determined by the variation that comes from common causes. It generally represents the best performance of the process itself. This is demonstracted when the process is being operated in a state of statistical control regardless of the specification.

中文版:
过程能力由造成变差的普通原因来确定,通常代表过程本身的最佳性能,当过程是在统计控制状态下运行时,过程能力可以得到证实,它与规范无关.

众所周知,过程能力Cp/Cpk计算需要规范的,为什么手册这样说呢?疑惑中......

诸位可以讨论一下吗?