There are two reasons your colours may print differently across jobs.
You may notice some differences in colour if you have ordered the same artwork but on different paper stocks, such as coated and uncoated papers, or adding a lamination. Colours on uncoated paper will “dry back” and give a slightly duller colour than on coated paper. Adding a lamination can also change how colours appear so please bear this in mind when placing your order.
Alternatively, both jobs may have been printed within tolerance but appear different. There are acceptable tolerances within the full-colour litho-print process, meaning colours in reprints may appear different to the original print job. So although your job will be printed within these tolerances, the end results may appear slightly different across reprints.