Why is the computed mass of a Magnesium nucleus greater than the sum of the mass of its nucleons?

Question

In mass defect, the computed mass of a stable nucleus is always less than the combined mass of its protons and neutrons. How come this isn't so with the Magnesium nucleus?

Answer 1

Magnesium has 3 stable isotopes,

12Mg24,  23.985 041 90 g/mol, 78.99%      

12Mg25,  24.985 837 02 g/mol, 10.00%  

12Mg26,  25.982 593 04 g/mol, 11.01%

Each isotope has an atomic mass less than the sum of the masses of its nucleons.

The average atomic mass of Magnesium is 24.3050 g/mol, which is greater than only 12Mg24's sum-of-nucleons.