Here's why:

* Atomic number represents the number of protons in an atom's nucleus. This number defines what element the atom is.

* Carbon is defined by having 6 protons.

* Carbon-14 and Carbon-12 are isotopes of carbon, meaning they have the same number of protons but different numbers of neutrons.

* Carbon-14 has 8 neutrons, while Carbon-12 has 6 neutrons.

So, while their mass numbers differ (14 for Carbon-14 and 12 for Carbon-12), their atomic numbers are the same.