Why does the energy required to remove an electron change as it does?

The mass of Cl on the periodic table is 35.45 atomic mass units. This is an average mass of the isotopes of chlorine. Most Cl atoms in nature have a relative mass of about 35 atomic mass units. Most of the remaining Cl isotopes have a relative mass of 37 atomic mass units.

The number 35.45u is the result of the natural mixture of Cl-35 and Cl-37, which produces the observed value.