Why is the mass in amu of a carbon-12 atom reported as 12.011 in the periodic table of the elements?

The mass of an element listed in the Periodic Table is the weighted average of all its naturally occurring isotopes.

Naturally occurring carbon is about

99 % carbon-12 (12.000 u) + 1 % carbon-13 (13.003 u).

That extra carbon-13 makes the average atomic mass greater than 12.000 u.