Ink Tolerance
Due to the ink tolerances involved in the four colour printing process, slight variance in finished printed colour is inevitable. This means that although your job will be printed within tolerance, and to ISO standards, the finished colour may turn out to be slightly different to expected, and may not be an exact match to the original colour in your design file. Subsequent reprints may differ in colour slightly from previous print runs, but this is perfectly normal and always within tolerance.
For jobs with multiple set-ups, such as booklets, there may be a variance in colour within the job itself, but this variance will be slight, if at all, and again within tolerance.