The different versions of an element have different isotopes, like Copper-63 and Carbon-65. The average of these two is not 64, because there are more Carbon-63 in the world than Carbon-65, 69.17% Copper-63 and 30.83% Copper-65, to be exact. By multiplying the atomic mass per unit of each one times their respective abundance in the world turned into a decimal, and adding these two products together, you het the atomic mass overall, which is 63.55
