Why is FeO (s) not formed in stoichiometric composition?

Iron(II) oxide or ferrous oxide is the inorganic compound with the formula FeO. This compound also refers to a family of related non-stoichiometric compounds, which are typically iron deficient with compositions ranging from Fe_0.84O to Fe_0.95O. This happens because of metal deficiency defect, a non-stoichiometric defect. In crystals of FeO, some Fe²⁺ cations are missing and the loss of positive charge is made up by the presence of required number of Fe³⁺ ions. Three Fe²⁺ ions are replaced by two Fe³⁺ ions to make up for the loss of positive charge. Hence, FeO is not formed in stoichiometric composition.