Why is the heat of combustion of sulphur equal to the heat of formation of SO2 and not SO3

By definition the heat of combustion for compound is equal to the energy that is released on complete oxidation of one mole that substance to its highest oxidation state. In case of sulphur, online I found that ∆H for $\ce{S8 + O2->SO2}$ is heat of combustion. Clearly + 4 is not the highest oxidation state of sulphur, then why is ∆H for $\ce{S8 + O2->SO3}$ not considered as heat of combustion


