“Isotopic MIXTURE” M-I-X-T-U-R-E ... meaning... more than one pure isotope.
Ok, and why should the periodic tables care about isotope mixtures? They already list the most stable isotope and when I’m dealing with a mixture, I can use this number against experimental data to derive the actual mixture ratio from the ratio of the expected vs actual weights.
By fudging the numbers, I have to look up the common isotope masses every time. Worthless.