Average bond Enthalpy is because all the similar bonds in a molecule do not have the same bond enthalpies.

Therefore, in polyatomic molecules term mean or average bond enthalpy is used. It is obtained by dividing total bond dissociation enthalpy by the number of bonds broken.

For water, the two OH bonds have different bond enthalpies.

This is the average bond enthalpy of O-H bond in water.

Thus, there is a difference in bond enthalpy of the O—H bond in ethanol (C₂H₅OH) and water.