You are correct that the relative atomic mass of an element, as listed on the periodic table, is usually not a whole number. The relative atomic mass is the weighted average of the masses of all the naturally occurring isotopes of an element, taking into account their relative abundances.
When calculating the relative atomic mass, the atomic mass of carbon-12 is indeed taken as a reference. However, the resulting relative atomic mass of an element is not necessarily a whole number because it is an average of the masses of different isotopes with different numbers of neutrons.
For example, let's consider chlorine (Cl). Chlorine has two stable isotopes: chlorine-35, which has 17 protons and 18 neutrons, and chlorine-37, which has 17 protons and 20 neutrons. The relative atomic mass of chlorine is listed as approximately 35.45 on the periodic table. This value is not a whole number because it is a weighted average of the masses of chlorine-35 and chlorine-37, taking into account their natural abundances.
So, the atomic mass of an element is not necessarily divisible by 1/12th of the atomic mass of carbon-12, due to the presence of isotopes and their respective abundances.