Question

Why actual atomic mass is greater than mass number?

Answer 1

The mass number is the integer sum of protons and neutrons for the most common isotope. Actual atomic mass=mass of electrons+mass of neutrons+mass of protons.

So actual atomic mass is greater than a mass number. however, the difference between them is negligible because the mass of the electron is $\frac{1}{1837}$ of protons.