Most elements contain a mixture of isotopes, each present in a different amount.
The term ‘weighted mean mass’ takes into account the different contribution of each isotope to the overall mass.
The contributions of all isotopes are combined to give the relative atomic mass – the weighted mean of the mass numbers.
Mass numbers are whole numbers because they are the number of protons plus the number of neutrons.
Relative atomic mass is an average mass, which takes into account all of the isotopes and their abundance. It is not always a whole number. However, on your data leaflet, apart from chlorine, the relative atomic masses have been rounded to whole numbers.