The atomic mass of carbon is commonly represented as 12.01 u, not 12.00 u, because carbon has multiple isotopes with different masses. The most abundant isotope of carbon is carbon-12, which has a mass of exactly 12 atomic mass units (u). However, there is also a small fraction of carbon-13, which has a mass of approximately 13 atomic mass units.
When calculating the average atomic mass of an element, the relative abundances of its isotopes are taken into account. Since carbon-12 is much more abundant than carbon-13, the average atomic mass of carbon is slightly higher than 12.00 u. The exact value of 12.01 u represents the weighted average of the masses of carbon-12 and carbon-13, taking into consideration their relative abundances.
The presence of carbon-13 in small amounts causes a slight increase in the average atomic mass, resulting in the value of 12.01 u. This value is more precise and reflects the natural abundance of carbon isotopes more accurately than a rounded value of 12.00 u would.