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.

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.