Explain the Discrepancy between carbon-12 and the natural mass of Carbon.?

Answer 1

Carbon 12 is the most abundant isotope of carbon, comprising 98.9% of all carbon known, but there are eight different isotopes of carbon:
Carbon 9 - six protons, 3 neutrons
Carbon 10 - six protons, 4 neutrons
Carbon 11 - six protons, 5 neutrons
*Carbon 12 - six protons, 6 neutrons; 98.9%, defined by convention with an atomic mass of 12.000000 g/mole
Carbon 13 - six protons, 7 neutrons; 1.1%
Carbon 14 - six protons, 8 neutrons
Carbon 15 - six protons, 9 neutrons
and Carbon 16 - six protons, 10 neutrons

When you take the weighted average of the atomic masses (the mass multiplied by its fraction) you find the listed mass for carbon of 12.011 g/mole, as shown below:

(12.0 * 0.989) + (13.003355 * 0.011) = 12.011036905 g/mole

Answer 2

Carbon 12 is the most common isotope of Carbon

The atomic mass you read [edit: on the periodic table] of Carbon (C) is the AVERAGE mass of all its isotopes.