Chemical energetics: explain question

A student estimates the standard enthalpy change of formation of ethanol, $\ce{C2H5OH}$, from mean bond energies. She lists every covalent bond in one molecule of ethanol and every covalent bond in the elements in their standard states, takes the values from a data table of mean bond energies, and uses

$$\Delta H = \sum E_b(\text{bonds broken}) - \sum E_b(\text{bonds formed})$$

She states that the value obtained is the standard enthalpy change of formation of ethanol.

Explain why this value will not be equal to the tabulated standard enthalpy change of formation of ethanol. In your answer, refer to standard conditions, the breaking and making of bonds, and the difference between exact and average bond energies.