No two things are ever exactly alike. There is "common" variation in everything. All processes have variation. There are normal differences in the process caused by heat changes, the rotation of a wheel, the squirt of a material, and many other things. These variables are hard to detect but they are always there in the process.
Another "special" variation might suddenly be introduced into the process by the sudden loss of power, by running out of material or by having a piece of equipment break. These "special" causes of variation are usually easily identified and corrected. This type of variation isn't an expected part of the process which is what makes it different to "common" variation.
Also see this Powerpoint .ppt Presentation information: Capability indices - Process Variation