The issue is usually in contamination of raw materials.
Similar to carbon-14 dating - the isotopic ratio of carbon in the atmosphere is maintained by cosmic radiation hitting the carbon-12 and carbon-13 atoms, but as soon as the carbon is incorporated into solid matter it ceases to be part of the process that renews the carbon-14 levels.
Similar to carbon-14 dating - the isotopic ratio of carbon in the atmosphere is maintained by cosmic radiation hitting the carbon-12 and carbon-13 atoms, but as soon as the carbon is incorporated into solid matter it ceases to be part of the process that renews the carbon-14 levels.