The brackets in the table mean that different sources of the element have different proportions of isotopes, so the mean atomic weights for specific deposits ma

The brackets in the table mean that different sources of the element have different proportions of isotopes, so the mean atomic weights for specific deposits may cover a range that differs from the overall mean for every deposit of the element ever measured. I.e. if you measure carbon from the upper atmosphere, it's going to have more C-14 in it, from cosmic rays flipping protons in N-14 to neutrons. And if you measure carbon buried for thousands of years, it's going to have less C-14, from natural decay. If you look at https://en.wikipedia.org/wiki/List_of_physical_constants https://en.wikipedia.org/wiki/List_of_physical_constants you can see that the parentheses are omitted from defined constants, and included for measured constants.