Why are atomic masses listed in the Periodic Table not whole numbers?

Answer 1

Two reasons:

1) All isotopic weights are based on the weight of carbon-12 being exactly 12.0000... amu. Every other weight is relative to it, so they are not necessarily whole numbers.
2) The weights listed in the table are the weighted averages for the stable isotopes of the element. Carbon's atomic weight in the table is 12.011, because even though C-12 is assigned a weight of 12.000, about 1% of naturally-occuring stable carbon is C-13, which is of course heavier, so that the weight in the table is higher than that of just C-12.

Answer 2

because there are various isotopes of those elements, versios of that element with different atomic masses, so they average up all the elements out there with the different atomic masses and write that one down, as the most common one found.