physical chemistry - Why is the relative atomic mass of carbon not exactly 12?

Relative atomic masses of atoms of all chemical elements are numbers without units, being the value of proportion compared to $\frac{1}{12}^\text{th}$ the mass of the carbon atom.

But the relative atomic mass of carbon is never 12! Instead, it is 12.01 or, more accurately, 12.011.

Why is this so?

Answer

Simply because the atomic mass is defined as 1/12 of the mass of ¹²C. Others isotopes of carbon (¹³C mostly, with an abundance of 1.1% approximately) account for an average atomic mass slightly above 12.