Why is the atomic mass of carbon 12.0111 amu and not just 12?

The masses given in periodic tables are weighted averages of all available isotopes not the actual mass.

The accurate mass of a proton is (1.0073 amu). The mass of a neutron is (1.0087 amu). The mass of C-13 is not 13 (13.0033548378 amu).

The isotopic abundances of C-12 and C-13 are 98.93% and 1.07%, respectively.

The atomic mass of carbon therefore is

(0.9893) (12 amu) + (0.0107) (13.00336) = 12.0107 amu