The atomic weights of most chemical elements aren’t constant

Last year, the International Union of Pure and Applied Chemistry (Iupac) revised the atomic weights of gadolinium, lutetium and zirconium. Gadolinium’s standard atomic weight changed from 157.250 to 157.249; lutetium’s went from 174.9668 to 174.96669, while zirconium’s went from 91.224 to 91.222. Those change are minuscule, so why bother?