Differentiate between atomic mass and mass number. Why are they usually different?

Mass number (A) is the total count of protons and neutrons in a specific atom, always a whole number. Atomic mass (or atomic weight) is the weighted average of the masses of all naturally occurring isotopes of an element, often a decimal number. They differ because atomic mass accounts for the varying abundances of isotopes and the slight mass defect from nuclear binding energy, whereas mass number is just a count of nucleons for a single isotope.