Why `NiO` and `FeO` are nonstroichiometric with the formula `Ni_(0.98)O` and `Fe_(0.95)O`?

Explanation

Some `Ni^(2+)` ions are replaced by `Ni^(3+)` ions, `3 Ni^(2+) -= 2 Ni^(3 +)` in `NiO` and likewise some `Fe^(2+)` ions are replaced by `Fe^(3+)` ions, `3 Fe^(2+) -= 2 Fe^(3+)` to maintain electrical neutrality.