The purpose of the control limits is to indicate when the variability of the process is so great that some special cause is likely to be operating. When a process observation exceeds the control limits a search for a special cause should be initiated. Continuous quality improvement is more concerned with analytical studies than enumerative ones. No two things in this world are exactly identical. Even two measurements of the same thing will differ if the measuring instrument is sufficiently precise.
All processes have outputs, upon which measurements can be taken.
Processes, then, must exhibit variability. Any process has an inherent level of natural variability. This may be small or large, acceptable or unacceptable, depending on the process. A well behaved process may be expected to fluctuate randomly within its natural limits of variability.