We may interpret the energy variable as free energy of formation _per mole of $\ce{O2}$_.  Thus, for instance, silicon would be favored to react with a limited amount of $\ce{O2}$ versus iron, because silica has a more negative free energy of formation _per mole of $\ce{O2}$_ than iron oxides -- even though $ce{Fe3O4}$ might be more negative _per mole of compound_ because $ce{Fe3O4}$ uses two moles of $\ce{O2}$ per mole of complound versus $\ce{SiO2}$ using one.  This, of course, is one of the main drivers behind silicon going into slag when we smelt and purify iron.