Why will you never find an atom that has an actual atomic mass equal to the elements average atomic mass?

Most all elements you see on the periodic table up to element 92 (uranium) are naturally occurring, and have at least two isotopes - atoms of the same element which contain the same number of protons, but a different number of neutrons.
Both protons and neutrons have an atomic mass of 1 "atomic mass unit" (amu). An atom containing 6 protons (carbon) can have 6, 7, or 8 neutrons. Respectively, they would have atomic masses of 12, 13, and 14 amu.

Over 99% of the carbon atoms found in nature have 6 neutrons (Carbon-12), a very small fraction have 8 neutrons (Carbon-14), and an even tinier fraction have 7 neutrons (Carbon-12). Taken as an average, the mass is 12.011 amu...just a bit over 12 amu.